mptcp: fix joined subflows with unblocking sk

Unblocking sockets used for outgoing connections were not containing
inet info about the initial connection due to a typo there: the value of
"err" variable is negative in the kernelspace.

This fixes the creation of additional subflows where the remote port has
to be reused if the other host didn't announce another one. This also
fixes inet_diag showing blank info about MPTCP sockets from unblocking
sockets doing a connect().

Fixes: 41be81a8

 ("mptcp: fix unblocking connect()")
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>