mptcp: sockopt: use new helper for TCP_DEFER_ACCEPT

mptcp_setsockopt_sol_tcp_defer() was doing the same thing as
mptcp_setsockopt_first_sf_only() except for the returned code in case of
error.

Ignoring the error is needed to mimic how TCP_DEFER_ACCEPT is handled
when used with "plain" TCP sockets.

The specific function for TCP_DEFER_ACCEPT can be replaced by the new
mptcp_setsockopt_first_sf_only() helper and errors can be ignored to
stay compatible with TCP. A bit of cleanup.

Suggested-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>