mptcp: use msk instead of mptcp_sk

Use msk instead of mptcp_sk(sk) in the functions where the variable
"msk = mptcp_sk(sk)" has been defined.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>