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

cmd/zhack/zhack.c

@@ -126,7 +126,8 @@ zhack_import(char *target, boolean_t readonly)
 	nvlist_t *props;
 	int error;
 
-	kernel_init(readonly ? FREAD : (FREAD | FWRITE));
+	kernel_init(readonly ? SPA_MODE_READ :
+	    (SPA_MODE_READ | SPA_MODE_WRITE));
 
 	dmu_objset_register_type(DMU_OST_ZFS, space_delta_cb);