Squashfs: Compute expected length from inode size rather than block length

Previously in squashfs_readpage() when copying data into the page
cache, it used the length of the datablock read from the filesystem
(after decompression).  However, if the filesystem has been corrupted
this data block may be short, which will leave pages unfilled.

The fix for this is to compute the expected number of bytes to copy
from the inode size, and use this to detect if the block is short.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Tested-by: Willy Tarreau <w@1wt.eu>
Cc: Анатолий Тросиненко <anatoly.trosinenko@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/squashfs/squashfs.h

@@ -72,7 +72,7 @@ void squashfs_copy_cache(struct page *, struct squashfs_cache_entry *, int,
 				int);
 
 /* file_xxx.c */
-extern int squashfs_readpage_block(struct page *, u64, int);
+extern int squashfs_readpage_block(struct page *, u64, int, int);
 
 /* id.c */
 extern int squashfs_get_id(struct super_block *, unsigned int, unsigned int *);