devfs: Abstract locking assertions

The conversion was largely mechanical: sed(1) with:

-e 's|mtx_assert(&devmtx, MA_OWNED)|dev_lock_assert_locked()|g'
-e 's|mtx_assert(&devmtx, MA_NOTOWNED)|dev_lock_assert_unlocked()|g'

The definitions of these abstractions in fs/devfs/devfs_int.h are the
only non-mechanical change.

No functional change.