bpf: Allow to change skb mark in test_run

allow to pass skb's mark field into bpf_prog_test_run ctx
for BPF_PROG_TYPE_SCHED_CLS prog type. that would allow
to test bpf programs which are doing decision based on this
field

Signed-off-by: Nikita V. Shirokov <tehnerd@tehnerd.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>