selftests: mptcp: more robust signal race test

selftests: mptcp: more robust signal race test

The in kernel MPTCP PM implementation can process a single
incoming add address option at any given time. In the
mentioned test the server can surpass such limit. Let the
setup cope with that allowing a faster add_addr retransmission.

Fixes: a88c9e496937 ("mptcp: do not block subflows creation on errors")
Fixes: f7efc7771eac ("mptcp: drop argument port from mptcp_pm_announce_addr")
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/254
Reported-and-tested-by: Matthieu Baerts <[email protected]>
Reviewed-by: Matthieu Baerts <[email protected]>
Signed-off-by: Paolo Abeni <[email protected]>
Signed-off-by: Mat Martineau <[email protected]>
Signed-off-by: David S. Miller <[email protected]>