block: Fix blk_validate_atomic_write_limits() build for arm32

For arm32, we get the following build warning:
 In file included from /tmp/next/build/include/linux/printk.h:10,
                  from /tmp/next/build/include/linux/kernel.h:31,
                  from /tmp/next/build/block/blk-settings.c:5:
 /tmp/next/build/block/blk-settings.c: In function 'blk_validate_atomic_write_limits':
 /tmp/next/build/include/asm-generic/div64.h:222:35: warning: comparison of distinct pointer types lacks a cast
   222 |         (void)(((typeof((n)) *)0) == ((uint64_t *)0));  \
       |                                   ^~

The divident for do_div() should be 64b, which it is not. Since we want to
check 2x unsigned ints, just use % operator. This allows us to drop the
chunk_sectors variable.

Fixes: 9da3d1e9

 ("block: Add core atomic write support")
Reported-by: Mark Brown <broonie@kernel.org>
Closes: https://lore.kernel.org/linux-next/b765d200-4e0f-48b1-a962-7dfa1c4aef9c@kernel.dk/T/#mbf067b1edd89c7f9d7dac6e258c516199953a108
Signed-off-by: John Garry <john.g.garry@oracle.com>
Link: https://lore.kernel.org/r/20240621183016.3092518-1-john.g.garry@oracle.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>