ipv6: remove nexthop_fib6_nh_bh()

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/d...