[XFS] Fix calculation of reserved AGs for inodes in 32-bit inode mode

Spotted by Roger Willcocks <willcor @at@ gmail.com>

SGI-PV: 944858
SGI-Modid: xfs-linux:xfs-kern:201213a

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