xfs: create helpers to scan an allocation group

Add some helpers to enable us to lock an AG's headers, create btree
cursors for all btrees in that allocation group, and clean up
afterwards.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

fs/xfs/scrub/scrub.c

@@ -44,6 +44,8 @@
 #include "scrub/scrub.h"
 #include "scrub/common.h"
 #include "scrub/trace.h"
+#include "scrub/scrub.h"
+#include "scrub/btree.h"
 
 /*
  * Online Scrub and Repair
@@ -141,6 +143,7 @@ xfs_scrub_teardown(
 	struct xfs_scrub_context	*sc,
 	int				error)
 {
+	xfs_scrub_ag_free(sc, &sc->sa);
 	if (sc->tp) {
 		xfs_trans_cancel(sc->tp);
 		sc->tp = NULL;
@@ -241,6 +244,7 @@ retry_op:
 	sc.sm = sm;
 	sc.ops = ops;
 	sc.try_harder = try_harder;
+	sc.sa.agno = NULLAGNUMBER;
 	error = sc.ops->setup(&sc, ip);
 	if (error)
 		goto out_teardown;