xfs: rearrange xfs_mod_sb() to avoid array subscript warning

gcc warns of an array subscript out of bounds in xfs_mod_sb().
The code is written in such a way that if the array subscript is
out of bounds, then it will assert fail. Rearrange the code to
avoid the bounds check warning.

Signed-off-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>