mptcp: drop unused sk in mptcp_get_options

The parameter 'sk' became useless since the code using it was dropped
from mptcp_get_options() in the commit 8d548ea1 ("mptcp: do not set
unconditionally csum_reqd on incoming opt"). Let's drop it.

Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>