e2fsck: fix journal block tag checksum verification

e2fsck: fix journal block tag checksum verification

Al Viro complained of a ton of bogosity with regards to the jbd2 block tag
header checksum.  This one checksum is 16 bits, so cut off the upper 16 bits
and treat it as a 16-bit value and don't mess around with be32* conversions.
Fortunately metadata checksumming is still "experimental" and not in a shipping
e2fsprogs, so there should be few users affected by this.

This is the e2fsprogs version of the kernel patch.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>