witness: don't warn about matrix inconsistencies without holding the mutex

Lock order checking is done without the witness mutex held, so multiple
threads that are racing to establish a new lock order may read matrix
entries that are in an inconsistent state. Don't print a warning in this
case, but instead just redo the check after taking the witness lock.

Differential Revision: https://reviews.freebsd.org/D2713
Reviewed by: jhb
MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division