compat: Always use own __ipv6_select_ident().

compat: Always use own __ipv6_select_ident().

If the ip fragmentation backport is enabled, we should always use our
own {,__}ipv6_select_ident(). This fixes the following issue on some
v3.19 kernels:

datapath/linux/ip6_output.c:93:12: error: conflicting types for
‘__ipv6_select_ident’
 static u32 __ipv6_select_ident(struct net *net, u32 hashrnd,

Reported-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Joe Stringer <joe@ovn.org>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Tested-by: Simon Horman <simon.horman@netronome.com>