Fix a bug in our RB_SCAN calls. A critical section is required to

Fix a bug in our RB_SCAN calls.  A critical section is required to
protect the RB tree against I/O completions.  Also do not abort a scan
if an I/O completion moves a buffer off the dirty list and onto the clean
list while we are trying to lock it.

There is a small chance that this bug could have resulted in dirty buffers
being left intact after being freed from the filesystem bitmap, which in
turn can cause filesystem corruption (dirbad and other panics).

Note that this was a DragonFly-only bug.