[XFS] endianess annotation for xfs_agfl_t. Trivial, xfs_agfl_t is always

used for ondisk values.

SGI-PV: 954580
SGI-Modid: xfs-linux-melb:xfs-kern:26553a

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Nathan Scott <nathans@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>