xfs: move some code around inside xfs_bmap_shift_extents

For the first right move we need to look up next_fsb.  That means
our last fsb that contains next_fsb must also be the current extent,
so take advantage of that by moving the code around a bit.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>

fs/xfs/libxfs/xfs_bmap.c

@@ -6127,7 +6127,6 @@ xfs_bmap_shift_extents(
 	ASSERT(xfs_isilocked(ip, XFS_IOLOCK_EXCL));
 	ASSERT(xfs_isilocked(ip, XFS_ILOCK_EXCL));
 	ASSERT(direction == SHIFT_LEFT || direction == SHIFT_RIGHT);
-	ASSERT(*next_fsb != NULLFSBLOCK || direction == SHIFT_RIGHT);
 
 	ifp = XFS_IFORK_PTR(ip, whichfork);
 	if (!(ifp->if_flags & XFS_IFEXTENTS)) {
@@ -6159,23 +6158,17 @@ xfs_bmap_shift_extents(
 	 * In case of first right shift, we need to initialize next_fsb
 	 */
 	if (*next_fsb == NULLFSBLOCK) {
-		gotp = xfs_iext_get_ext(ifp, total_extents - 1);
+		ASSERT(direction == SHIFT_RIGHT);
+
+		current_ext = total_extents - 1;
+		gotp = xfs_iext_get_ext(ifp, current_ext);
 		xfs_bmbt_get_all(gotp, &got);
 		*next_fsb = got.br_startoff;
 		if (stop_fsb > *next_fsb) {
 			*done = 1;
 			goto del_cursor;
 		}
-	}
-
-	/* Lookup the extent index at which we have to stop */
-	if (direction == SHIFT_RIGHT) {
-		gotp = xfs_iext_bno_to_ext(ifp, stop_fsb, &stop_extent);
-		/* Make stop_extent exclusive of shift range */
-		stop_extent--;
-	} else
-		stop_extent = total_extents;
-
+	} else {
 		/*
 		 * Look up the extent index for the fsb where we start shifting. We can
 		 * henceforth iterate with current_ext as extent list changes are locked
@@ -6190,13 +6183,24 @@ xfs_bmap_shift_extents(
 			*done = 1;
 			goto del_cursor;
 		}
+	}
 
-	/* some sanity checking before we finally start shifting extents */
-	if ((direction == SHIFT_LEFT && current_ext >= stop_extent) ||
-	     (direction == SHIFT_RIGHT && current_ext <= stop_extent)) {
+	/* Lookup the extent index at which we have to stop */
+	if (direction == SHIFT_RIGHT) {
+		xfs_iext_bno_to_ext(ifp, stop_fsb, &stop_extent);
+		/* Make stop_extent exclusive of shift range */
+		stop_extent--;
+		if (current_ext <= stop_extent) {
 			error = -EIO;
 			goto del_cursor;
 		}
+	} else {
+		stop_extent = total_extents;
+		if (current_ext >= stop_extent) {
+			error = -EIO;
+			goto del_cursor;
+		}
+	}
 
 	while (nexts++ < num_exts) {
 		error = xfs_bmse_shift_one(ip, whichfork, offset_shift_fsb,