xfs: remove xfs_inode_ag_iterator_flags

Combine xfs_inode_ag_iterator_flags and xfs_inode_ag_iterator_tag into a
single wrapper function since there's only one caller of the _flags
variant.

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