selftests: netdevsim: Test route offload failure notifications

Add cases to verify that when debugfs variable "fail_route_offload" is
set, notification with "rt_offload_failed" flag is received.

Extend the existing cases to verify that when sysctl
"fib_notify_on_flag_change" is set to 2, the kernel emits notifications
only for failed route installation.

$ ./fib_notifications.sh
TEST: IPv4 route addition				[ OK ]
TEST: IPv4 route deletion				[ OK ]
TEST: IPv4 route replacement				[ OK ]
TEST: IPv4 route offload failed				[ OK ]
TEST: IPv6 route addition				[ OK ]
TEST: IPv6 route deletion				[ OK ]
TEST: IPv6 route replacement				[ OK ]
TEST: IPv6 route offload failed				[ OK ]

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