net: fib_rules: Factorise fib_newrule() and fib_delrule().

net: fib_rules: Factorise fib_newrule() and fib_delrule().

fib_nl_newrule() / fib_nl_delrule() is the doit() handler for
RTM_NEWRULE / RTM_DELRULE but also called from vrf_newlink().

Currently, we hold RTNL on both paths but will not on the former.

Also, we set dev_net(dev)->rtnl to skb->sk in vrf_fib_rule() because
fib_nl_newrule() / fib_nl_delrule() fetch net as sock_net(skb->sk).

Let's Factorise the two functions and pass net and rtnl_held flag.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250207072502.87775-6-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>