ipv6: Move common init code for rt6_info to a new function rt6_info_init()

Introduce rt6_info_init() to do the common init work for
'struct rt6_info' (after calling dst_alloc).

It is a prep work to fix the rt6_info init logic in the
ip6_blackhole_route().

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Cc: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Julian Anastasov <ja@ssi.bg>
Cc: Phil Sutter <phil@nwl.cc>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Signed-off-by: David S. Miller <davem@davemloft.net>