NFSD: unregister filesystem in case genl_register_family() fails

With rpc_status netlink support, unregister of register_filesystem()
was missed in case of genl_register_family() fails.

Correcting it by making new label.

Fixes: bd9d6a3e ("NFSD: add rpc_status netlink support")
Cc: stable@vger.kernel.org
Signed-off-by: Maninder Singh <maninder1.s@samsung.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>