INVARIANTS: treat LA_LOCKED as the same of LA_XLOCKED in mtx_assert.

The Linux lockdep API assumes LA_LOCKED semantic in lockdep_assert_held(),
meaning that either a shared lock or write lock is Ok. On the other hand,
the timeout code uses lc_assert() with LA_XLOCKED, and we need both to
work.

For mutexes, because they can not be shared (this is unique among all lock
classes, and it is unlikely that we would add new lock class anytime soon),
it is easier to simply extend mtx_assert to handle LA_LOCKED there, despite
the change itself can be viewed as a slight abstraction violation.

Reviewed by: mjg, cem, jhb
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D21362