net: increase preallocated size of nlmsg to accomodate for IFLA_STATS64

When more data is stuffed into an nlmsg than initially projected, an
extra allocation needs to be done. Reserve enough for IFLA_STATS64 so
that this does not to needlessy happen.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: David S. Miller <davem@davemloft.net>