e2undo: cast dptr to blk64_t to retrieve block number

e2undo: cast dptr to blk64_t to retrieve block number

A 32-bit s390 build was failing on a 64-bit s390x host, when
make check failed e2undo tests, like this:

md5sum before mke2fs 922c8a591c882dbdd1a381d18547cfd5
using mke2fs to test e2undo
Overwriting existing filesystem; this can be undone using the command:
    e2undo /tmp/mke2fs-tmp.EM9XjmTA81.e2undo /tmp/tmp.EM9XjmTA81

md5sum after mke2fs cbf32fb6c3db45280ad013f42ac294f1
Replayed transaction of size 32768 at location 0
Replayed transaction of size 32768 at location 0
Replayed transaction of size 32768 at location 0
Replayed transaction of size 32768 at location 0
Replayed transaction of size 0 at location 0
md5sum after e2undo 31b4e14307c5b7ccce5b8d300c2ad5f1

Note the "at location 0" for the block number.

A proper cast in e2undo.c fixes this up.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>