resize2fs: fix 32-bit overflow issue which can corrupt 64-bit file systems

resize2fs: fix 32-bit overflow issue which can corrupt 64-bit file systems

Fix a 32-bit overflow bug caused by a missing blk64_t cast which can
cause the block bitmap to get corrupted when doing an off-line resize
of a 64-bit file system.

This problem can be reproduced as follows:

rm -f foo.img; touch foo.img
truncate -s 8T foo.img
mke2fs -F -t ext4 -O 64bit foo.img
e2fsck -f foo.img
truncate -s 21T foo.img
resize2fs foo.img
e2fsck -fy foo.img

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>