xfs: minor cleanup for xfs_get_blocks

Simplify the control flow a bit in preparation for O_ATOMIC-related
changes.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>