mptcp: use mptcp_get_ext helper

Use mptcp_get_ext() helper defined in protocol.h instead of open-coding
it in mptcp_sendmsg_frag().

Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231025-send-net-next-20231025-v1-6-db8f25f798eb@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>