of the struct plex, which tore apart the mutex linked lists when the
plex table was expanded. Now we maintain a pool of mutexes (currently
32) to be shared by all plexes. This is still a lot better than the
splhigh() method used in other architectures.
Add and clarify comments.
Approved by: re (jhb)
DIOCGMEDIASIZE instead.
The partition type check has been XXX'ed out since we cannot expect
that a BSD disklabel with a type field be available on all platforms.
