afs: Fix infinite loop found by xfstest generic/676

In AFS, a directory is handled as a file that the client downloads and
parses locally for the purposes of performing lookup and getdents
operations.  The in-kernel afs filesystem has a number of functions that
do this.

A directory file is arranged as a series of 2K blocks divided into
32-byte slots, where a directory entry occupies one or more slots, plus
each block starts with one or more metadata blocks.

When parsing a block, if the last slots are occupied by a dirent that
occupies more than a single slot and the file position points at a slot
that's not the initial one, the logic in afs_dir_iterate_block() that
skips over it won't advance the file pointer to the end of it.  This
will cause an infinite loop in getdents() as it will keep retrying that
block and failing to advance beyond the final entry.

Fix this by advancing the file pointer if the next entry will be beyond
it when we skip a block.

This was found by the generic/676 xfstest but can also be triggered with
something like:

	~/xfstests-dev/src/t_readdir_3 /xfstest.test/z 4000 1

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Marc Dionne <marc.dionne@auristor.com>
Tested-by: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Link: http://lore.kernel.org/r/165391973497.110268.2939296942213894166.stgit@warthog.procyon.org.uk/Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/afs/dir.c

@@ -463,8 +463,11 @@ static int afs_dir_iterate_block(struct afs_vnode *dvnode,
 		}
 
 		/* skip if starts before the current position */
-		if (offset < curr)
+		if (offset < curr) {
+			if (next > curr)
+				ctx->pos = blkoff + next * sizeof(union afs_xdr_dirent);
 			continue;
+		}
 
 		/* found the next entry */
 		if (!dir_emit(ctx, dire->u.name, nlen,