Fix invalid context bug

In the module unload path the vm_file_cache was being destroyed
under a spin lock.  Because this operation might sleep it was
possible, although very very unlikely, that this could result
in a deadlock.

This issue was indentified by using a Linux debug kernel and
has been fixed by moving the kmem_cache_destroy() out from under
the spin lock.  There is no need to lock this operation here.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes zfsonlinux/zfs#771

module/spl/spl-vnode.c

@@ -845,13 +845,12 @@ spl_vn_fini(void)
 		leaked++;
 	}
 
-	kmem_cache_destroy(vn_file_cache);
-	vn_file_cache = NULL;
 	spin_unlock(&vn_file_lock);
 
 	if (leaked > 0)
 		SWARN("Warning %d files leaked\n", leaked);
 
+	kmem_cache_destroy(vn_file_cache);
 	kmem_cache_destroy(vn_cache);
 
 	SEXIT;