mptcp: cleanup SOL_TCP handling

commit 7f71a337 upstream.

Most TCP-level socket options get an integer from user space, and
set the corresponding field under the msk-level socket lock.

Reduce the code duplication moving such operations in the common code.

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Stable-dep-of: bd11dc4f ("mptcp: fix full TCP keep-alive support")
[ Without TCP_NOTSENT_LOWAT support, as it is not in this version, see
  commit 29b5e5ef ("mptcp: implement TCP_NOTSENT_LOWAT support") ]
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>