Merge branch 'ipv4-preliminary-work-for-per-netns-rtnl'

Eric Dumazet says:

====================
ipv4: preliminary work for per-netns RTNL

Inspired by 9b8ca04854fd ("ipv4: avoid quadratic behavior in
FIB insertion of common address") and per-netns RTNL conversion
started by Kuniyuki this week.

ip_fib_check_default() can use RCU instead of a shared spinlock.

fib_info_lock can be removed, RTNL is already used.

fib_info_devhash[] can be removed in favor of a single
pointer in net_device.
====================

Link: https://patch.msgid.link/20241004134720.579244-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>