Add zfs_file_* interface, remove vnodes

Provide a common zfs_file_* interface which can be implemented on all 
platforms to perform normal file access from either the kernel module
or the libzpool library.

This allows all non-portable vnode_t usage in the common code to be 
replaced by the new portable zfs_file_t.  The associated vnode and
kobj compatibility functions, types, and macros have been removed
from the SPL.  Moving forward, vnodes should only be used in platform
specific code when provided by the native operating system.

Reviewed-by: Sean Eric Fagan <sef@ixsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9556

include/sys/dmu.h

@@ -49,6 +49,7 @@
 #include <sys/zio_compress.h>
 #include <sys/zio_priority.h>
 #include <sys/uio.h>
+#include <sys/zfs_file.h>
 
 #ifdef	__cplusplus
 extern "C" {
@@ -1071,7 +1072,7 @@ void dmu_traverse_objset(objset_t *os, uint64_t txg_start,
     dmu_traverse_cb_t cb, void *arg);
 
 int dmu_diff(const char *tosnap_name, const char *fromsnap_name,
-    struct vnode *vp, offset_t *offp);
+    zfs_file_t *fp, offset_t *offp);
 
 /* CRC64 table */
 #define	ZFS_CRC64_POLY	0xC96C5795D7870F42ULL	/* ECMA-182, reflected form */