libxfs: reset dirty buffer priority on lookup

libxfs: reset dirty buffer priority on lookup

When a buffer on the dirty MRU is looked up and found, we remove the
buffer from the MRU. However, we've already set the priority of the
buffer to "dirty" so when we are done with it it will go back on the
dirty buffer MRU regardless of whether it needs to or not.

Hence when we move a buffer to a the dirty MRU, record the old
priority and restore it when we remove the buffer from the MRU on
lookup. This will prevent us from putting fixed, now writeable
buffers back on the dirty MRU and allow the cache routine to write,
shake and reclaim the buffers once they are clean.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>