sctp_diag: Fix T3_rtx timer export

The asoc's timer value is not kept in asoc->timeouts array but in it's
primary transport instead.

Furthermore, we must export the timer only if it is pending, otherwise
the value will underrun when stored in an unsigned variable and
user space will only see a very large timeout value.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>