badblocks: fix mis-printed error from block size check

badblocks: fix mis-printed error from block size check

block_size is parsed as an unsigned int from parse_uint(), so retain it
as such until _after_ it has been constrained to a size within INT_MAX.

Lower level code still requires this to be an int, so cast to int for
anything below main().

Before:
$ misc/badblocks -w -b 4294967295 -c 1 /tmp/testfile.bin
misc/badblocks: Invalid block size: -1

After:
$ misc/badblocks -w -b 4294967295 -c 1 /tmp/testfile.bin
misc/badblocks: Invalid block size: 4294967295

Signed-off-by: Corey Hickey <bugfood-c@fatooh.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>