Fix a MP lock race. The MP locking state can change when lwkt_chktokens()

Fix a MP lock race.  The MP locking state can change when lwkt_chktokens()
returns a failure.  The state is not retained by an interrupt because we
are trying to set it for the new thread rather then the current thread, and
the current thread's mpcount could be 0.

Change ASSERT_MP_LOCK_HELD() to supply a thread for reporting purposes in
the KASSERT.

Reported-by: David Rhodus <sdrhodus@gmail.com>