Set cwd to '/' for the process executing insmod.

Ricardo has pointed out that under Solaris the cwd is set to '/'
during module load, while under Linux it is set to the callers cwd.
To handle this cleanly I've reworked the module *_init()/_exit()
macros so they call a *_setup()/_cleanup() function when any SPL
dependent module is loaded or unloaded.  This gives us a chance to
perform any needed modification of the process, in this case changing
the cwd.  It also handily provides a way to avoid creating wrapper
init()/exit() functions because the Solaris and Linux prototypes
differ slightly.  All dependent modules should now call the spl
helper macros spl_module_{init,exit}() instead of the native linux
versions.

Unfortunately, it appears that under Linux there has been no consistent
API in the kernel to set the cwd in a module.  Because of this I have
had to add more autoconf magic than I'd like.  However, what I have
done is correct and has been tested on RHEL5, SLES11, FC11, and CHAOS
kernels.

In addition, I have change the rootdir type from a 'void *' to the
correct 'vnode_t *' type.  And I've set rootdir to a non-NULL value.

include/linux/module_compat.h

@@ -0,0 +1,35 @@
+#ifndef _SPL_MODULE_COMPAT_H
+#define _SPL_MODULE_COMPAT_H
+
+#include <linux/module.h>
+
+#define spl_module_init(init_fn)                                        \
+static int                                                              \
+spl_##init_fn(void)                                                     \
+{                                                                       \
+	int rc;                                                         \
+	                                                                \
+	spl_setup();                                                    \
+	rc = init_fn();                                                 \
+                                                                        \
+	return rc;                                                      \
+}                                                                       \
+                                                                        \
+module_init(spl_##init_fn)
+
+#define spl_module_exit(exit_fn)                                        \
+static void                                                             \
+spl_##exit_fn(void)                                                     \
+{                                                                       \
+	int rc;                                                         \
+                                                                        \
+	rc = exit_fn();                                                 \
+	spl_cleanup();                                                  \
+	if (rc)                                                         \
+		printk(KERN_ERR "SPL: Failure %d unloading "            \
+		       "dependent module\n", rc);                       \
+}                                                                       \
+                                                                        \
+module_exit(spl_##exit_fn)
+
+#endif /* _SPL_MODULE_COMPAT_H */

include/sys/sysmacros.h

@@ -144,6 +144,8 @@ extern int p0;
 /* Missing misc functions */
 extern int highbit(unsigned long i);
 extern uint32_t zone_get_hostid(void *zone);
+extern void spl_setup(void);
+extern void spl_cleanup(void);
 
 #define makedevice(maj,min) makedev(maj,min)
 

include/sys/types.h

@@ -18,6 +18,7 @@ extern "C" {
 #include <linux/workqueue_compat.h>
 #include <linux/kallsyms_compat.h>
 #include <linux/mutex_compat.h>
+#include <linux/module_compat.h>
 
 #ifndef HAVE_UINTPTR_T
 typedef unsigned long			uintptr_t;

include/sys/vnode.h

@@ -215,6 +215,7 @@ extern int vn_getattr(vnode_t *vp, vattr_t *vap, int flags, void *x3, void *x4);
 extern int vn_fsync(vnode_t *vp, int flags, void *x3, void *x4);
 extern file_t *vn_getf(int fd);
 extern void vn_releasef(int fd);
+extern int vn_set_pwd(const char *filename);
 
 int vn_init(void);
 void vn_fini(void);
@@ -241,7 +242,7 @@ vn_putpage(vnode_t *vp, offset_t off, ssize_t size,
 #define getf					vn_getf
 #define releasef				vn_releasef
 
-extern void *rootdir;
+extern vnode_t *rootdir;
 
 #ifdef  __cplusplus
 }