smbd: Simplify fsp_lease_update()

We don't need a share mode lock from a data dependency point of view
anymore, the leases data moved to leases.tdb. However, from a
coherency point of view it's probably wise to do this under a share
mode lock.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
