netconsole: add oops_only module option

Some people wants to log only oops messages via netconsole,
(this is also why netoops was invented)
so add a module option for netconsole. This can be tuned
via /sys/module/netconsole/parameters/oops_only at run time
as well.

Cc: David Miller <davem@davemloft.net>
Signed-off-by: Cong Wang <amwang@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>