selftests: netdevsim: Add test cases for route deletion failure

selftests: netdevsim: Add test cases for route deletion failure

Add IPv4 and IPv6 test cases that ensure that we are not leaking a
reference on the nexthop device when we are unable to delete its
associated route.

Without the fix in a previous patch ("netdevsim: fib: Fix reference
count leak on route deletion failure") both test cases get stuck,
waiting for the reference to be released from the dummy device [1][2].

[1]
unregister_netdevice: waiting for dummy1 to become free. Usage count = 5
leaked reference.
 fib_check_nh+0x275/0x620
 fib_create_info+0x237c/0x4d30
 fib_table_insert+0x1dd/0x1d20
 inet_rtm_newroute+0x11b/0x200
 rtnetlink_rcv_msg+0x43b/0xd20
 netlink_rcv_skb+0x15e/0x430
 netlink_unicast+0x53b/0x800
 netlink_sendmsg+0x945/0xe40
 ____sys_sendmsg+0x747/0x960
 ___sys_sendmsg+0x11d/0x190
 __sys_sendmsg+0x118/0x1e0
 do_syscall_64+0x34/0x80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

[2]
unregister_netdevice: waiting for dummy1 to become free. Usage count = 5
leaked reference.
 fib6_nh_init+0xc46/0x1ca0
 ip6_route_info_create+0x1167/0x19a0
 ip6_route_add+0x27/0x150
 inet6_rtm_newroute+0x161/0x170
 rtnetlink_rcv_msg+0x43b/0xd20
 netlink_rcv_skb+0x15e/0x430
 netlink_unicast+0x53b/0x800
 netlink_sendmsg+0x945/0xe40
 ____sys_sendmsg+0x747/0x960
 ___sys_sendmsg+0x11d/0x190
 __sys_sendmsg+0x118/0x1e0
 do_syscall_64+0x34/0x80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>