xfs: use generic inode iterator in xfs_qm_dqrele_all_inodes

Use xfs_inode_ag_iterator instead of opencoding the inode walk in the
quota code.  Mark xfs_inode_ag_iterator and xfs_sync_inode_valid non-static
to allow using them from the quota code.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Josef 'Jeff' Sipek <jeffpc@josefsipek.net>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>