xfs: simulate per-AG reservations being critically low

Create an error injection point that enables us to simulate being
critically low on per-AG block reservations.  This should enable us to
simulate this specific ENOSPC condition so that we can test falling back
to a regular file copy.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
diff --git a/fs/xfs/xfs_error.h b/fs/xfs/xfs_error.h
index 8d8e1b07..05f8666 100644
--- a/fs/xfs/xfs_error.h
+++ b/fs/xfs/xfs_error.h
@@ -95,7 +95,8 @@
 #define XFS_ERRTAG_REFCOUNT_CONTINUE_UPDATE		24
 #define XFS_ERRTAG_REFCOUNT_FINISH_ONE			25
 #define XFS_ERRTAG_BMAP_FINISH_ONE			26
-#define XFS_ERRTAG_MAX					27
+#define XFS_ERRTAG_AG_RESV_CRITICAL			27
+#define XFS_ERRTAG_MAX					28
 
 /*
  * Random factors for above tags, 1 means always, 2 means 1/2 time, etc.
@@ -127,6 +128,7 @@
 #define XFS_RANDOM_REFCOUNT_CONTINUE_UPDATE		1
 #define XFS_RANDOM_REFCOUNT_FINISH_ONE			1
 #define XFS_RANDOM_BMAP_FINISH_ONE			1
+#define XFS_RANDOM_AG_RESV_CRITICAL			4
 
 #ifdef DEBUG
 extern int xfs_error_test_active;