mptcp: sockopt: make sync_socket_options propagate SOCK_KEEPOPEN

mptcp: sockopt: make sync_socket_options propagate SOCK_KEEPOPEN

Users reported a scenario where MPTCP connections that were configured
with SO_KEEPALIVE prior to connect would fail to enable their keepalives
if MTPCP fell back to TCP mode.

After investigating, this affects keepalives for any connection where
sync_socket_options is called on a socket that is in the closed or
listening state.  Joins are handled properly. For connects,
sync_socket_options is called when the socket is still in the closed
state.  The tcp_set_keepalive() function does not act on sockets that
are closed or listening, hence keepalive is not immediately enabled.
Since the SO_KEEPOPEN flag is absent, it is not enabled later in the
connect sequence via tcp_finish_connect.  Setting the keepalive via
sockopt after connect does work, but would not address any subsequently
created flows.

Fortunately, the fix here is straight-forward: set SOCK_KEEPOPEN on the
subflow when calling sync_socket_options.

The fix was valdidated both by using tcpdump to observe keepalive
packets not being sent before the fix, and being sent after the fix.  It
was also possible to observe via ss that the keepalive timer was not
enabled on these sockets before the fix, but was enabled afterwards.

Fixes: 1b3e7ede1365 ("mptcp: setsockopt: handle SO_KEEPALIVE and SO_PRIORITY")
Cc: stable@vger.kernel.org
Signed-off-by: Krister Johansen <kjlx@templeofstupid.com>
Reviewed-by: Geliang Tang <geliang@kernel.org>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/aL8dYfPZrwedCIh9@templeofstupid.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>