When user requests to resolve an output route, the kernel synthesizes
an skb where the relevant parameters (e.g., source address) are set. The
skb is then passed to ip_route_output_key_hash_rcu() which might call
into the flow dissector in case a multipath route was hit and a nexthop
needs to be selected based on the multipath hash.

Since both 'skb->dev' and 'skb->sk' are not set, a warning is triggered
in the flow dissector [1]. The warning is there to prevent codepaths
from silently falling back to the standard flow dissector instead of the
BPF one.

Therefore, instead of removing the warning, set 'skb->dev' to the
loopback device, as its not used for anything but resolving the correct
namespace.

[1]
WARNING: CPU: 1 PID: 24819 at net/core/flow_dissector.c:764 __skb_flow_dissect+0x314/0x16b0
...
RSP: 0018:ffffa0df41fdf650 EFLAGS: 00010246
RAX: 0000000000000000 RBX: ffff8bcded232000 RCX: 0000000000000000
RDX: ffffa0df41fdf7e0 RSI: ffffffff98e415a0 RDI: ffff8bcded232000
RBP: ffffa0df41fdf760 R08: 0000000000000000 R09: 0000000000000000
R10: ffffa0df41fdf7e8 R11: ffff8bcdf27a3000 R12: ffffffff98e415a0
R13: ffffa0df41fdf7e0 R14: ffffffff98dd2980 R15: ffffa0df41fdf7e0
FS:  00007f46f6897680(0000) GS:ffff8bcdf7a80000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055933e95f9a0 CR3: 000000021e636000 CR4: 00000000001006e0
Call Trace:
 fib_multipath_hash+0x28c/0x2d0
 ? fib_multipath_hash+0x28c/0x2d0
 fib_select_path+0x241/0x32f
 ? __fib_lookup+0x6a/0xb0
 ip_route_output_key_hash_rcu+0x650/0xa30
 ? __alloc_skb+0x9b/0x1d0
 inet_rtm_getroute+0x3f7/0xb80
 ? __alloc_pages_nodemask+0x11c/0x2c0
 rtnetlink_rcv_msg+0x1d9/0x2f0
 ? rtnl_calcit.isra.24+0x120/0x120
 netlink_rcv_skb+0x54/0x130
 rtnetlink_rcv+0x15/0x20
 netlink_unicast+0x20a/0x2c0
 netlink_sendmsg+0x2d1/0x3d0
 sock_sendmsg+0x39/0x50
 ___sys_sendmsg+0x2a0/0x2f0
 ? filemap_map_pages+0x16b/0x360
 ? __handle_mm_fault+0x108e/0x13d0
 __sys_sendmsg+0x63/0xa0
 ? __sys_sendmsg+0x63/0xa0
 __x64_sys_sendmsg+0x1f/0x30
 do_syscall_64+0x5a/0x120
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Fixes: d0e13a14

 ("flow_dissector: lookup netns by skb->sk if skb->dev is NULL")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>