nexthop: Fix nexthop_num_path for blackhole nexthops

Donald reported this sequence:
  ip next add id 1 blackhole
  ip next add id 2 blackhole
  ip ro add 1.1.1.1/32 nhid 1
  ip ro add 1.1.1.2/32 nhid 2

would cause a crash. Backtrace is:

[  151.302790] general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN PTI
[  151.304043] CPU: 1 PID: 277 Comm: ip Not tainted 5.3.0-rc5+ #37
[  151.305078] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.1-1 04/01/2014
[  151.306526] RIP: 0010:fib_add_nexthop+0x8b/0x2aa
[  151.307343] Code: 35 f7 81 48 8d 14 01 c7 02 f1 f1 f1 f1 c7 42 04 01 f4 f4 f4 48 89 f2 48 c1 ea 03 65 48 8b 0c 25 28 00 00 00 48 89 4d d0 31 c9 <80> 3c 02 00 74 08 48 89 f7 e8 1a e8 53 ff be 08 00 00 00 4c 89 e7
[  151.310549] RSP: 0018:ffff888116c27340 EFLAGS: 00010246
[  151.311469] RAX: dffffc0000000000 RBX: ffff8881154ece00 RCX: 0000000000000000
[  151.312713] RDX: 0000000000000004 RSI: 0000000000000020 RDI: ffff888115649b40
[  151.313968] RBP: ffff888116c273d8 R08: ffffed10221e3757 R09: ffff888110f1bab8
[  151.315212] R10: 0000000000000001 R11: ffff888110f1bab3 R12: ffff888115649b40
[  151.316456] R13: 0000000000000020 R14: ffff888116c273b0 R15: ffff888115649b40
[  151.317707] FS:  00007f60b4d8d800(0000) GS:ffff88811ac00000(0000) knlGS:0000000000000000
[  151.319113] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  151.320119] CR2: 0000555671ffdc00 CR3: 00000001136ba005 CR4: 0000000000020ee0
[  151.321367] Call Trace:
[  151.321820]  ? fib_nexthop_info+0x635/0x635
[  151.322572]  fib_dump_info+0xaa4/0xde0
[  151.323247]  ? fib_create_info+0x2431/0x2431
[  151.324008]  ? napi_alloc_frag+0x2a/0x2a
[  151.324711]  rtmsg_fib+0x2c4/0x3be
[  151.325339]  fib_table_insert+0xe2f/0xeee
...

fib_dump_info incorrectly has nhs = 0 for blackhole nexthops, so it
believes the nexthop object is a multipath group (nhs != 1) and ends
up down the nexthop_mpath_fill_node() path which is wrong for a
blackhole.

The blackhole check in nexthop_num_path is leftover from early days
of the blackhole implementation which did not initialize the device.
In the end the design was simpler (fewer special case checks) to set
the device to loopback in nh_info, so the check in nexthop_num_path
should have been removed.

Fixes: 430a0491 ("nexthop: Add support for nexthop groups")
Reported-by: Donald Sharp <sharpd@cumulusnetworks.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>