Don't recheck about timeout == 0 during mutex locking

Don't recheck about timeout == 0 during mutex locking

If the timeout wasn't zero, it can only become zero if we return from
futex() with a non-timeout reason but subsequently expires while we're
recalculating something.

A side effect is that we try-lock a non-recursive mutex exactly
once. Before this change, we'd fastTryLock() twice even with
timeout == 0.

Change-Id: I0af09fc2a84669a683a843fcf1513203b075dfb7
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>