ipv6: remove nexthop_fib6_nh_bh()

commit ef1148d4 upstream.

After blamed commit, nexthop_fib6_nh_bh() and nexthop_fib6_nh()
are the same.

Delete nexthop_fib6_nh_bh(), and convert /proc/net/ipv6_route
to standard rcu to avoid this splat:

[ 5723.180080] WARNING: suspicious RCU usage
[ 5723.180083] -----------------------------
[ 5723.180084] include/net/nexthop.h:516 suspicious rcu_dereference_check() usage!
[ 5723.180086]
other info that might help us debug this:

[ 5723.180087]
rcu_scheduler_active = 2, debug_locks = 1
[ 5723.180089] 2 locks held by cat/55856:
[ 5723.180091] #0: ffff9440a582afa8 (&p->lock){+.+.}-{3:3}, at: seq_read_iter (fs/seq_file.c:188)
[ 5723.180100] #1: ffffffffaac07040 (rcu_read_lock_bh){....}-{1:2}, at: rcu_lock_acquire (include/linux/rcupdate.h:326)
[ 5723.180109]
stack backtrace:
[ 5723.180111] CPU: 14 PID: 55856 Comm: cat Tainted: G S        I        6.3.0-dbx-DEV #528
[ 5723.180115] Call Trace:
[ 5723.180117]  <TASK>
[ 5723.180119] dump_stack_lvl (lib/dump_stack.c:107)
[ 5723.180124] dump_stack (lib/dump_stack.c:114)
[ 5723.180126] lockdep_rcu_suspicious (include/linux/context_tracking.h:122)
[ 5723.180132] ipv6_route_seq_show (include/net/nexthop.h:?)
[ 5723.180135] ? ipv6_route_seq_next (net/ipv6/ip6_fib.c:2605)
[ 5723.180140] seq_read_iter (fs/seq_file.c:272)
[ 5723.180145] seq_read (fs/seq_file.c:163)
[ 5723.180151] proc_reg_read (fs/proc/inode.c:316 fs/proc/inode.c:328)
[ 5723.180155] vfs_read (fs/read_write.c:468)
[ 5723.180160] ? up_read (kernel/locking/rwsem.c:1617)
[ 5723.180164] ksys_read (fs/read_write.c:613)
[ 5723.180168] __x64_sys_read (fs/read_write.c:621)
[ 5723.180170] do_syscall_64 (arch/x86/entry/common.c:?)
[ 5723.180174] entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:120)
[ 5723.180177] RIP: 0033:0x7fa455677d2a

Fixes: 09eed119

 ("neighbour: switch to standard rcu, instead of rcu_bh")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20230510154646.370659-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>