LU-12600 tgt: shortio size should be unsigned

LU-12600 tgt: shortio size should be unsigned

The short_io_size value is accepting unsigned values from
req_capsule_get_size, and so needs to be unsigned as well.

If it's not, it's possible for the short_io_size memcopy to
act on an incorrect value and cause memory corruption.

Reported-by: Alibaba Cloud <yunye.ry@alibaba-inc.com>
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I043e314cd43a7b40519f951a605fa5a36ff91dcf
Reviewed-on: https://review.whamcloud.com/35653
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>