s3/smbd: update exclusive oplock optimisation to the lease area

s3/smbd: update exclusive oplock optimisation to the lease area

Update an optimisation in update_num_read_oplocks() that checks for
exclusive oplocks to the lease area.

The idea of the optimisation is to avoid expensive db queries in
brlock.tdb if we *know* we're the only open.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=12766

Signed-off-by: Ralph Boehme <slow@samba.org>
Signed-off-by: Jeremy Allison <jra@samba.org>
Autobuild-User(master): Ralph Böhme <slow@samba.org>
Autobuild-Date(master): Sat May  6 22:58:47 CEST 2017 on sn-devel-144