net-memcg: pass in gfp_t mask to mem_cgroup_charge_skmem()

Add gfp_t mask as an input parameter to mem_cgroup_charge_skmem(),
to give more control to the networking stack and enable it to change
memcg charging behavior. In the future, the networking stack may decide
to avoid oom-kills when fallbacks are more appropriate.

One behavior change in mem_cgroup_charge_skmem() by this patch is to
avoid force charging by default and let the caller decide when and if
force charging is needed through the presence or absence of
__GFP_NOFAIL.

Signed-off-by: Wei Wang <weiwan@google.com>
Reviewed-by: Shakeel Butt <shakeelb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/sock.h

@@ -2400,6 +2400,11 @@ static inline gfp_t gfp_any(void)
 	return in_softirq() ? GFP_ATOMIC : GFP_KERNEL;
 }
 
+static inline gfp_t gfp_memcg_charge(void)
+{
+	return in_softirq() ? GFP_NOWAIT : GFP_KERNEL;
+}
+
 static inline long sock_rcvtimeo(const struct sock *sk, bool noblock)
 {
 	return noblock ? 0 : sk->sk_rcvtimeo;