fix a bug where we access a bread buffer after we have brelse'd it...
The kernel normally didn't unmap/context switch away before we accessed
the buffer most of the time, but under heavy I/O pressure and lots of
mount/unmounting this would cause a fault on nofault panic...

Reviewed by:	dteske
Approved by:	re (kib)
Sponsored by:	Vicor
MFC after:	3 days
NULL stale pointers (should be a no-op as they should no longer be
used)...

Reviewed by:	dteske
Approved by:	re (kib)
Sponsored by:	Vicor
MFC after:	3 days