Add DSCP support for network QoS to iscsi initiator.

Allow the DSCP codepoint also to be configurable
for the traffic in the direction from the initiator
to the target, such that writes and any requests
are also treated in the appropriate QoS class.

Reviewed by: mav
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D26714