smbd: Add share_mode_do_locked()

smbd: Add share_mode_do_locked()

This is made for efficient locking of share mode records in
locking.tdb. Right now we already need that when accessing leases.tdb,
and soon it will be required for brlock.tdb as well. It does not give
direct access to the parsed share mode entry, but the record is
available for dbwrap_watched_wakeup() within downgrade_lease().

It can be freely nested with get_share_mode_lock calls, the record
will be shared and proper nesting should be checked.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>