tdb: Fix bug 11381, deadlock

tdb: Fix bug 11381, deadlock

This fixes a deadlock in tdb that is a bad interaction between tdb_lockall
and tdb_traverse. This deadlock condition has been around even before
tdb mutexes, it's just that the kernel fcntl EDEADLK detection protected
us from this ABBA lock condition to become a real deadlock stalling
processes. With tdb mutexes, this deadlock protection is gone, so we do
lock dead.

This patch glosses over this particular ABBA condition, making tdb with
mutexes behave the same as tdb without mutexes. Admittedly this is no
real fix, but it works around a real user's problem.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=11381
Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>
(cherry picked from commit 1061a9cafda7d73ebcd2f74e69e74f4adc485d5d)