l2tp: fix missing print session offset info

l2tp: fix missing print session offset info

commit 820da5357572715c6235ba3b3daa2d5b43a1198f upstream.

Report offset parameter in L2TP_CMD_SESSION_GET command if
it has been configured by userspace

Fixes: 309795f4bec ("l2tp: Add netlink control API for L2TP")
Reported-by: Jianlin Shi <jishi@redhat.com>
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2:
 - Use NLA_PUT_U16, consistent with the rest of the function
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>