mptcp: add mptcp_destroy_common helper

This patch added a new helper named mptcp_destroy_common containing the
shared code between mptcp_destroy() and mptcp_sock_destruct().

Suggested-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Geliang Tang <geliangtang@gmail.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>