witness(4): Print stack of prior observed lock order on reversal

The first time Witness observes a lock order between two locks, it records
the caller's stack. On detected reversal, print out that previous observed
stack. It is quite possible that the reported "LOR" is the correct
ordering, and the violation was the observed earlier ordering.

Reviewed by: mjg
Differential Revision: https://reviews.freebsd.org/D26070