Squashfs: add sanity checks to id reading at mount time

Fsfuzzer generates corrupted filesystems which throw a warn_on in
kmalloc.  One of these is due to a corrupted superblock no_ids field.
Fix this by checking that the number of bytes to be read (and allocated)
does not extend into the next filesystem structure.

Also add a couple of other sanity checks of the mount-time id table
structures.

Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>

fs/squashfs/xattr.h

@@ -31,6 +31,7 @@ static inline __le64 *squashfs_read_xattr_id_table(struct super_block *sb,
 		u64 start, u64 *xattr_table_start, int *xattr_ids)
 {
 	ERROR("Xattrs in filesystem, these will be ignored\n");
+	*xattr_table_start = start;
 	return ERR_PTR(-ENOTSUPP);
 }