[XFS] Fix racy access to pb_flags. pagebuf_rele() modified pb_flags after

the pagebuf had been unlocked if the buffer was delwri. At high load, this
could result in a race when the superblock was being synced that would
result the flags being incorrect and the iodone functions being executed
incorrectly. This then leads to iclog callback failures or AIL list
corruptions resulting in filesystem shutdowns.

SGI-PV: 923981
SGI-Modid: xfs-linux:xfs-kern:23616a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>