[XFS] Reduce endian flipping in alloc_btree, same as was done for

ialloc_btree.

SGI-PV: 955302
SGI-Modid: xfs-linux-melb:xfs-kern:26910a

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Nathan Scott <nathans@sgi.com>
Signed-off-by: Tim Shimmin <tes@sgi.com>