xfs: create iterator error codes

Currently, xfs doesn't have generic error codes defined for "stop
iterating"; we just reuse the XFS_BTREE_QUERY_* return values.  This
looks a little weird if we're not actually iterating a btree index.
Before we start adding more iterators, we should create general
XFS_ITER_{CONTINUE,ABORT} return values and define the XFS_BTREE_QUERY_*
ones from that.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
diff --git a/fs/xfs/libxfs/xfs_alloc.c b/fs/xfs/libxfs/xfs_alloc.c
index ae2d91b..372ad55 100644
--- a/fs/xfs/libxfs/xfs_alloc.c
+++ b/fs/xfs/libxfs/xfs_alloc.c
@@ -3137,7 +3137,7 @@ xfs_alloc_has_record(
 
 /*
  * Walk all the blocks in the AGFL.  The @walk_fn can return any negative
- * error code or XFS_BTREE_QUERY_RANGE_ABORT.
+ * error code or XFS_ITER_*.
  */
 int
 xfs_agfl_walk(