stand: Remove const from zfs_lookup's zfsmount argument

Dynamic system attribute layout can require modifications to the mount
structure on lookup. Drop the const to allow that.

Sponsored by:		Netflix
Differential Revision:	https://reviews.freebsd.org/D60260

stand/libsa/zfs/zfsimpl.c

@@ -3845,7 +3845,7 @@ struct obj_list {
  * Lookup a file and return its dnode.
  */
 static int
-zfs_lookup(const struct zfsmount *mount, const char *upath,
+zfs_lookup(struct zfsmount *mount, const char *upath,
     dnode_phys_t *dnode, uint64_t *objnum_out)
 {
 	int rc;