brd: Switch rd_size to unsigned long

Currently rd_size was int which lead to overflow and bogus device size
once the requested ramdisk size was 1 TB or more. Although these days
ramdisks with 1 TB size are mostly a mistake, the days when they are
useful are not far.

Reported-by: Bart Van Assche <bart.vanassche@sandisk.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>