Convert filemon_lock and struct filemon* lock to sx(9), rather than a
self-rolled reader-writer lock, and hold it for the entire time needed.
At least filemon_lock_write() was not checking for active readers when
it would successfully return with the write lock "held". This led to
a race with reading entries from filemon_inuse as they were removed. This
could be seen with QUEUE_MACRO_DEBUG enabled, causing -1 to be read as an
entry rather than a valid struct filemon*.
Fixing filemon_lock_write() to check readers was insufficient to fix the
races.
sx(9) was used as the lock could be held while taking proctree_lock and sleeping
in fo_write.
Sponsored by: EMC / Isilon Storage Division
MFC after: 2 weeks
