The device busy flag was essentially a custom sleepable lock, which was
invisible to WITNESS. Replace it with an sx lock. This simplifies and
removes some of the PCM macros, and gives us WITNESS coverage. PCM_LOCK
stays for the paths that need a non-sleepable lock.
Name both locks after what they are rather than after the device unit,
so they are distinguishable in WITNESS.
No functional change intended.
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation