xfs: introduce xfs_dialloc_roll()

xfs: introduce xfs_dialloc_roll()

Source kernel commit: aececc9f8dec92a25c84a3378021636ce58d72dc

Introduce a helper to make the on-disk inode allocation rolling
logic clearer in preparation of the following cleanup.

[sandeen: update xfsprogs struct xfs_trans to match]

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>