badblocks: fix operation with large-ish block sizes and/or counts

badblocks: fix operation with large-ish block sizes and/or counts

test_rw() and test_nd() need to allocate two or three times the product
of the block size and the block counts. This can overflow the signed int
type of block_size and result in allocate_buffer() being called with a
value smaller than intended. Once that buffer is written to, badblocks
segfaults.

Since allocate_buffer() accepts a size_t, change the input validation to
use SIZE_MAX and cast accordingly when calculating the argument.

Fixing the segfault allows larger values to be passed to read() and
write(); these need to be cast to size_t as well in order to avoid a
signed integer overflow causing failure, in which case badblocks would
fall back to testing a single block at once.

Before:
$ misc/badblocks -w -b 4096 -c 524288 -e 1 -s -v /tmp/testfile.bin
Checking for bad blocks in read-write mode
From block 0 to 524287
Segmentation fault

$ misc/badblocks -n -b 4096 -c 524288 -e 1 -s -v /tmp/testfile.bin
Checking for bad blocks in non-destructive read-write mode
From block 0 to 524287
Checking for bad blocks (non-destructive read-write test)
Segmentation fault

After:
$ misc/badblocks -w -b 4096 -c 524288 -e 1 -s -v /tmp/testfile.bin
Checking for bad blocks in read-write mode
From block 0 to 524287
Testing with pattern 0xaa: done
Reading and comparing: done
Testing with pattern 0x55: done
Reading and comparing: done
Testing with pattern 0xff: done
Reading and comparing: done
Testing with pattern 0x00: done
Reading and comparing: done
Pass completed, 0 bad blocks found. (0/0/0 errors)

$ misc/badblocks -n -b 4096 -c 524288 -e 1 -s -v /tmp/testfile.bin
Checking for bad blocks in non-destructive read-write mode
From block 0 to 524287
Checking for bad blocks (non-destructive read-write test)
Testing with random pattern: done
Pass completed, 0 bad blocks found. (0/0/0 errors)

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