[XFS] endianess annotations for xfs_bmbt_key Trivial as there are no

incore users.

SGI-PV: 954580
SGI-Modid: xfs-linux-melb:xfs-kern:26561a

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