ipv6: fib6: avoid indirect calls from fib6_rule_lookup

ipv6: fib6: avoid indirect calls from fib6_rule_lookup

It was reported that a considerable amount of cycles were spent on the
expensive indirect calls on fib6_rule_lookup. This patch introduces an
inline helper called pol_route_func that uses the indirect_call_wrappers
to avoid the indirect calls.

This patch saves around 50ns per call.

Performance was measured on the receiver by checking the amount of
syncookies that server was able to generate under a synflood load.

Traffic was generated using trafgen[1] which was pushing around 1Mpps on
a single queue. Receiver was using only one rx queue which help to
create a bottle neck and make the experiment rx-bounded.

These are the syncookies generated over 10s from the different runs:

Whithout the patch:
TcpExtSyncookiesSent            3553749            0.0
TcpExtSyncookiesSent            3550895            0.0
TcpExtSyncookiesSent            3553845            0.0
TcpExtSyncookiesSent            3541050            0.0
TcpExtSyncookiesSent            3539921            0.0
TcpExtSyncookiesSent            3557659            0.0
TcpExtSyncookiesSent            3526812            0.0
TcpExtSyncookiesSent            3536121            0.0
TcpExtSyncookiesSent            3529963            0.0
TcpExtSyncookiesSent            3536319            0.0

With the patch:
TcpExtSyncookiesSent            3611786            0.0
TcpExtSyncookiesSent            3596682            0.0
TcpExtSyncookiesSent            3606878            0.0
TcpExtSyncookiesSent            3599564            0.0
TcpExtSyncookiesSent            3601304            0.0
TcpExtSyncookiesSent            3609249            0.0
TcpExtSyncookiesSent            3617437            0.0
TcpExtSyncookiesSent            3608765            0.0
TcpExtSyncookiesSent            3620205            0.0
TcpExtSyncookiesSent            3601895            0.0

Without the patch the average is 354263 pkt/s or 2822 ns/pkt and with
the patch the average is 360738 pkt/s or 2772 ns/pkt which gives an
estimate of 50 ns per packet.

[1] http://netsniff-ng.org/

Changelog since v1:
 - Change ordering in the ICW (Paolo Abeni)

Cc: Luigi Rizzo <lrizzo@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Reported-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Brian Vazquez <brianvv@google.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>