Fix a sign bug causing invalid memory access in the ino_paths ioctl.

To see the problem, create many hardlinks to the same file (120 should do it),
then look up paths by inode with:

  ls -i
  btrfs inspect inode-resolve -v $ino /mnt/btrfs

I noticed the memory layout of the fspath->val data had some irregularities
(some unnecessary gaps that stop appearing about halfway),
so I'm not sure there aren't any bugs left in it.

fs/btrfs/backref.c

@@ -1186,7 +1186,7 @@ static char *ref_to_path(struct btrfs_root *fs_root,
 	int slot;
 	u64 next_inum;
 	int ret;
-	s64 bytes_left = size - 1;
+	s64 bytes_left = ((s64)size) - 1;
 	struct extent_buffer *eb = eb_in;
 	struct btrfs_key found_key;
 	int leave_spinning = path->leave_spinning;