ipv4: remove fib_info_lock

ipv4: remove fib_info_lock

After the prior patch, fib_info_lock became redundant
because all of its users are holding RTNL.

BH protection is not needed.

Remove the READ_ONCE()/WRITE_ONCE() annotations around fib_info_cnt,
since it is protected by RTNL.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://patch.msgid.link/20241004134720.579244-4-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>