Squashfs: add sanity checks to fragment reading at mount time

Fsfuzzer generates corrupted filesystems which throw a warn_on in
kmalloc.  One of these is due to a corrupted superblock fragments 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 fragment table
structures.

Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>
diff --git a/fs/squashfs/squashfs.h b/fs/squashfs/squashfs.h
index 3b705f1..647b81b 100644
--- a/fs/squashfs/squashfs.h
+++ b/fs/squashfs/squashfs.h
@@ -57,7 +57,7 @@
 /* fragment.c */
 extern int squashfs_frag_lookup(struct super_block *, unsigned int, u64 *);
 extern __le64 *squashfs_read_fragment_index_table(struct super_block *,
-				u64, unsigned int);
+				u64, u64, unsigned int);
 
 /* id.c */
 extern int squashfs_get_id(struct super_block *, unsigned int, unsigned int *);