Fix two issues with lockmgr(9) LK_CAN_SHARE() test, which determines
whether the shared request for already shared-locked lock could be
granted.  Both problems result in the exclusive locker starvation.

The concurrent exclusive request is indicated by either
LK_EXCLUSIVE_WAITERS or LK_EXCLUSIVE_SPINNERS flags.  The reverse
condition, i.e. no exclusive waiters, must check that both flags are
cleared.

Add a flag LK_NODDLKTREAT for shared lock request to indicate that
current thread guarantees that it does not own the lock in shared
mode.  This turns back the exclusive lock starvation avoidance code;
see man page update for detailed description.

Use LK_NODDLKTREAT when doing lookup(9).

Reported and tested by:	pho
No objections from:	attilio
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
When non-forced unmount or remount rw->ro is performed, writes on UFS
are not suspended.  In particular, on the SU-enabled vulumes, there is
no reason why, between the call to softdep_flushfiles() and
softdep_waitidle(), SU work items cannot be queued.

Correct the condition to trigger the panic by only checking when
forced operation is done.  Convert direct panic() call into KASSERT(),
there is no invalid on-disk data structures directly involved, so
follow the usual debugging vs. non-debugging approach.

Reported and tested by:	pho
Reviewed by:	mckusick
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week