Revert "md: change mddev 'chunk_sectors' from int to unsigned"

This reverts commit 6ffeb1c3.

It causes problems :(

Reported-by: Dave Jones <davej@codemonkey.org.uk>
Reported-by: Mike Snitzer <snitzer@redhat.com>
Cc: Song Liu <songliubraving@fb.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Linus Torvalds <torvalds@linux-foundation.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>