MFC r305078 (by emaste):

iscsi_initiator: make logout_req::reason unsigned char

Previously this reported an error from Clang 3.9.0: implict conversion
from 'int' to 'char' changes value from 128 to -128.

Discussed with: dim, trasz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7699