[XFS] consolidate some code in xfs_page_state_convert The unmapped buffer

case is very similar to delayed and unwritten extends. Reorganize the code
to share some code for these cases.

SGI-PV: 947118
SGI-Modid: xfs-linux-melb:xfs-kern:203827a

Signed-off-by: Christoph Hellwig <hch@sgi.com>
Signed-off-by: Nathan Scott <nathans@sgi.com>