netns: read NETNSA_NSID as s32 attribute in rtnl_net_getid()

NETNSA_NSID is signed. Use nla_get_s32() to avoid confusion.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>