net: fib_rules: Split fib_nl2rule().

net: fib_rules: Split fib_nl2rule().

We will move RTNL down to fib_nl_newrule() and fib_nl_delrule().

Some operations in fib_nl2rule() require RTNL: fib_default_rule_pref()
and __dev_get_by_name().

Let's split the RTNL parts as fib_nl2rule_rtnl().

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/20250207072502.87775-4-kuniyu@amazon.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>