Fix an incorrect assertion in witness.

Fix an incorrect assertion in witness.

The number of available lock list entries for a thread is LOCK_CHILDCOUNT,
and each entry can record up to LOCK_NCHILDREN locks. When iterating over
the locks held by a thread, a bound on the loop index is therefore given
by LOCK_CHILDCOUNT * LOCK_NCHILDREN; WITNESS_COUNT is an unrelated
constant.

Reviewed by:	jhb
MFC after:	1 week
Sponsored by:	EMC / Isilon Storage Division
Differential Revision:	https://reviews.freebsd.org/D2974