neigh: fix setting of default gc_* values

This patch fixes bug introduced by:
commit 1d4c8c29
"neigh: restore old behaviour of default parms values"

The thing is that in neigh_sysctl_register, extra1 and extra2 which were
previously set for NEIGH_VAR_GC_* are overwritten. That leads to
nonsense int limits for gc_* variables. So fix this by not touching
extra* fields for gc_* variables.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>