mptcp: sockopt: drop redundant tcp_getsockopt

mptcp: sockopt: drop redundant tcp_getsockopt

tcp_getsockopt() is called twice in mptcp_getsockopt_first_sf_only() in
different conditions, which makes the code a bit redundant.

The first call to tcp_getsockopt() when the first subflow exists can be
replaced by going to a new label "get" before the second call.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250719-net-next-mptcp-tcp_maxseg-v2-1-8c910fbc5307@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>