When upgrading the shared lock to an exclusive lock, if we discover that the exclusive lock is already held, then we call panic. Don't clobber internal lock state before panic'ing. This change improves debugging if this case were to happen.
When upgrading the shared lock to an exclusive lock, if we discover
that the exclusive lock is already held, then we call panic. Don't
clobber internal lock state before panic'ing. This change improves
debugging if this case were to happen.

Submitted by:	Mohan Srinivasan mohans at yahoo-inc dot com
Reviewed by:	rwatson

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