xfs: fix regression in "cleanup xfs_dir2_block_getdents"

Commit 263dde86 ("xfs: cleanup xfs_dir2_block_getdents") introduced
a getdents regression, when it converted the pointer arithmetics to
offset calculations: offset is updated in the loop already for the next
iteration, but the updated offset value is used incorrectly in two
places, where we should have used the not-yet-updated value.

This caused for example "git clean -ffdx" failures to cleanup certain
directory structures when running in a container.

Fix the regression by making sure we use proper offset in the loop body.
Thanks to Christoph Hellwig for suggestion how to best fix the code.

Cc: Christoph Hellwig <hch@lst.de>
Fixes: 263dde86 ("xfs: cleanup xfs_dir2_block_getdents")
Signed-off-by: Tommi Rantala <tommi.t.rantala@nokia.com>
Reviewed-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>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

fs/xfs/xfs_dir2_readdir.c

@@ -147,7 +147,7 @@ xfs_dir2_block_getdents(
 	xfs_off_t		cook;
 	struct xfs_da_geometry	*geo = args->geo;
 	int			lock_mode;
-	unsigned int		offset;
+	unsigned int		offset, next_offset;
 	unsigned int		end;
 
 	/*
@@ -173,9 +173,10 @@ xfs_dir2_block_getdents(
 	 * Loop over the data portion of the block.
 	 * Each object is a real entry (dep) or an unused one (dup).
 	 */
-	offset = geo->data_entry_offset;
 	end = xfs_dir3_data_end_offset(geo, bp->b_addr);
-	while (offset < end) {
+	for (offset = geo->data_entry_offset;
+	     offset < end;
+	     offset = next_offset) {
 		struct xfs_dir2_data_unused	*dup = bp->b_addr + offset;
 		struct xfs_dir2_data_entry	*dep = bp->b_addr + offset;
 		uint8_t filetype;
@@ -184,14 +185,15 @@ xfs_dir2_block_getdents(
 		 * Unused, skip it.
 		 */
 		if (be16_to_cpu(dup->freetag) == XFS_DIR2_DATA_FREE_TAG) {
-			offset += be16_to_cpu(dup->length);
+			next_offset = offset + be16_to_cpu(dup->length);
 			continue;
 		}
 
 		/*
 		 * Bump pointer for the next iteration.
 		 */
-		offset += xfs_dir2_data_entsize(dp->i_mount, dep->namelen);
+		next_offset = offset +
+			xfs_dir2_data_entsize(dp->i_mount, dep->namelen);
 
 		/*
 		 * The entry is before the desired starting point, skip it.