s3:smbd: protect smbd_do_unlocking() with share_mode_do_locked_vfs_allowed()

s3:smbd: protect smbd_do_unlocking() with share_mode_do_locked_vfs_allowed()

share_mode_do_locked() will be make static soon.

Here we just want to avoid concurrent access to brlock.tdb
in order to maintain the lock order, we're not interested in the
locking.tdb content at all, expect that there's at least one
entry and we want to wake potential watchers.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15125

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>