[XFS] Simplify pagebuf_rele Remove a conditional that can not be true

anymore and simplify the final put path a little

SGI-PV: 908809
SGI-Modid: xfs-linux:xfs-kern:200790a

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