MAC: improve consistency in error handling

Whenever mac_syncache_init() returns an error, ensure that
*label = NULL. This simplifies the error handling by the caller.

Reviewed by: rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D46701

(cherry picked from commit 3f2792166aeed4baf07d351bcb12a9d196c443eb)