net: namespace: perform strict checks also for doit handlers

Make RTM_GETNSID's doit handler use strict checks when
NETLINK_F_STRICT_CHK is set.

v2: - don't check size >= sizeof(struct rtgenmsg) (Nicolas).

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>