Disable background bitmap writes. They appear to cause at least two race

Disable background bitmap writes.  They appear to cause at least two race
conditions:  First, on MP systems even an LK_NOWAIT lock may block,
invalidating flags checks done just prior to the lock attempt.  Second, on
both MP and UP systems, the original buffer (origbp) may be modified during
the completion of a background write without its lock being held and these
modifications can race against mainline code that is also modifying the same
buffer with the lock held.

Eventually the problem background bitmap writes solved will be solved more
generally by implementing page COWing durign device I/O to avoid stalls on
pages undergoing write I/O.