set SO_REUSEADDR for lnst-slave's listening socket

If LNST crashes for some reason user cannot restart it due to "Address
already in use" error. Setting socket option SO_REUSEADDR resolves this.

Signed-off-by: Jan Tluka <jtluka@redhat.com>
Signed-off-by: Jiri Pirko <jiri@resnulli.us>