Add blockdev_size() support for OpenBSD

Add blockdev_size() support for OpenBSD

This commit is OpenBSD version of 1116dc04 in 2015.
This is exactly the same as the previous commit for NetBSD.

OpenBSD still has block device as a file type, thus it differs
from FreeBSD/DragonFlyBSD regarding FIO_HAVE_CHARDEV_SIZE.

Note that fio doesn't compile on OpenBSD 5.9 (hits lex error),
but this this commit should still be good since this function
alone compiles on OpenBSD.

Signed-off-by: Tomohiro Kusumi <kusumi.tomohiro@gmail.com>
Signed-off-by: Jens Axboe <axboe@fb.com>