selftests/bpf: check bpf verifier log buffer usage works for HW offload

Make netdevsim print a message to the BPF verifier log buffer when a
program is offloaded.

Then use this message in hardware offload selftests to make sure that
using this buffer actually prints the message to the console for
eBPF hardware offload.

The message is appended after the last instruction is processed with the
verifying function from netdevsim. Output looks like the following:

    $ tc filter add dev foo ingress bpf obj sample_ret0.o \
        sec .text verbose skip_sw

    Prog section '.text' loaded (5)!
     - Type:         3
     - Instructions: 2 (0 over limit)
     - License:

    Verifier analysis:

    0: (b7) r0 = 0
    1: (95) exit
    [netdevsim] Hello from netdevsim!
    processed 2 insns, stack depth 0

"verbose" flag is required to see it in the console since netdevsim does
not throw an error after printing the message.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>