tdb: add run-fcntl-deadlock test

tdb: add run-fcntl-deadlock test

This verifies the F_RDLCK => F_WRLCK upgrade logic in the kernel
for conflicting locks.

This is a standalone test to check the traverse_read vs.
allrecord_lock/prepare_commit interaction.

This is based on the example from
https://lists.samba.org/archive/samba-technical/2017-April/119861.html
from Douglas Bagnall <douglas.bagnall@catalyst.net.nz> and Volker Lendecke <vl@samba.org>.

Signed-off-by: Stefan Metzmacher <metze@samba.org>
Reviewed-by: Garming Sam <garming@catalyst.net.nz>