Squashfs: fix corruption checks in squashfs_lookup()

The dir_count and size fields when read from disk are sanity
checked for correctness.  However, the sanity checks only check the
values are not greater than expected.  As dir_count and size were
incorrectly defined as signed ints, this can lead to corrupted values
appearing as negative which are not trapped.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>

fs/squashfs/namei.c

@@ -147,7 +147,8 @@ static struct dentry *squashfs_lookup(struct inode *dir, struct dentry *dentry,
 	struct squashfs_dir_entry *dire;
 	u64 block = squashfs_i(dir)->start + msblk->directory_table;
 	int offset = squashfs_i(dir)->offset;
-	int err, length, dir_count, size;
+	int err, length;
+	unsigned int dir_count, size;
 
 	TRACE("Entered squashfs_lookup [%llx:%x]\n", block, offset);