Update utsname support

Modify the code to use the utsname() kernel function rather than
a global variable.  This results is cleaner more portable code
because utsname() is already provided by the kernel and can be
easily emulated in user space via uname(2).  This means that it
will behave consistently in both contexts.

This is also has the benefit that it allows the removal of a few
_KERNEL pre-processor conditions.  And it also is a pre-requisite
for a proper FUSE port because we need to provide a valid utsname.

Finally, it allows us to remove this functionality from the SPL
and all the related compatibility code.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #2757

include/sys/zfs_context.h

@@ -67,6 +67,7 @@
 #include <sys/ctype.h>
 #include <sys/disp.h>
 #include <linux/dcache_compat.h>
+#include <linux/utsname_compat.h>
 
 #else /* _KERNEL */
 
@@ -117,6 +118,7 @@
 #include <sys/fm/fs/zfs.h>
 #include <sys/sunddi.h>
 #include <sys/debug.h>
+#include <sys/utsname.h>
 
 /*
  * Stack
@@ -673,6 +675,9 @@ extern int ddi_strtoul(const char *str, char **nptr, int base,
 extern int ddi_strtoull(const char *str, char **nptr, int base,
     u_longlong_t *result);
 
+typedef struct utsname	utsname_t;
+extern utsname_t *utsname(void);
+
 /* ZFS Boot Related stuff. */
 
 struct _buf {