xfs: avoid harmless gcc-7 warnings

gcc-7 flags the use of integer math inside of a condition
as a potential bug:

fs/xfs/xfs_bmap_util.c: In function 'xfs_swap_extents_check_format':
fs/xfs/xfs_bmap_util.c:1619:8: error: '<<' in boolean context, did you mean '<' ? [-Werror=int-in-bool-context]
fs/xfs/xfs_bmap_util.c:1629:8: error: '<<' in boolean context, did you mean '<' ? [-Werror=int-in-bool-context]

There is already a helper function for testing the di_forkoff
field for zero, so let's use that instead to shut up the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-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>