fsck.cramfs: Fix bus error on broken file system.

fsck.cramfs: Fix bus error on broken file system.

The utility fsck.cramfs is prone to a bus error on file systems for
big endian systems with non-standard header sizes. While calculating
the crc32 checksum, it does not properly handle a possible offset
for bootcodes, resulting in out of boundary access of mmap'ed area.

You can trigger the issue with the following commands:

$ mkdir -p cramfs-poc/root/subdir
$ cd cramfs-poc
$ mkfs.cramfs -p -N big root cramfs
$ echo -ne \\00\\x4c | dd of=cramfs bs=1 seek=518 count=2 conv=notrunc
$ fsck.cramfs cramfs

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>