net: fib_rules: Convert RTM_DELRULE to per-netns RTNL.

fib_nl_delrule() is the doit() handler for RTM_DELRULE but also called
from vrf_newlink() in case something fails in vrf_add_fib_rules().

In the latter case, RTNL is already held and the 4th arg is true.

Let's hold per-netns RTNL in fib_delrule() if rtnl_held is false.

Now we can place ASSERT_RTNL_NET() in call_fib_rule_notifiers().

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/20250207072502.87775-9-kuniyu@amazon.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>