xfs: make staging file forks explicit

Don't open-code "-1" for whichfork when we're creating a staging btree
for a repair; let's define an actual symbol to make grepping and
understanding easier.
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/xfs/libxfs/xfs_bmap_btree.c

@@ -614,7 +614,7 @@ xfs_bmbt_stage_cursor(
 	cur = xfs_bmbt_init_common(mp, NULL, ip, XFS_DATA_FORK);
 
 	/* Don't let anyone think we're attached to the real fork yet. */
-	cur->bc_ino.whichfork = -1;
+	cur->bc_ino.whichfork = XFS_STAGING_FORK;
 	xfs_btree_stage_ifakeroot(cur, ifake);
 	return cur;
 }

fs/xfs/libxfs/xfs_types.h

@@ -80,11 +80,13 @@ typedef void *		xfs_failaddr_t;
 /*
  * Inode fork identifiers.
  */
-#define	XFS_DATA_FORK	0
-#define	XFS_ATTR_FORK	1
-#define	XFS_COW_FORK	2
+#define XFS_STAGING_FORK	(-1)	/* fake fork for staging a btree */
+#define	XFS_DATA_FORK		(0)
+#define	XFS_ATTR_FORK		(1)
+#define	XFS_COW_FORK		(2)
 
 #define XFS_WHICHFORK_STRINGS \
+	{ XFS_STAGING_FORK, 	"staging" }, \
 	{ XFS_DATA_FORK, 	"data" }, \
 	{ XFS_ATTR_FORK,	"attr" }, \
 	{ XFS_COW_FORK,		"cow" }