mptcp: use msk_owned_by_me helper

The helper msk_owned_by_me() is defined in protocol.h, so use it instead
of sock_owned_by_me().

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: David S. Miller <davem@davemloft.net>