bpf: Fix bpf_prog_test_run_xdp logic after incorrect merge resolution

bpf: Fix bpf_prog_test_run_xdp logic after incorrect merge resolution

During recent net into net-next merge ([0]) a piece of old logic ([1]) got
reintroduced accidentally while resolving merge conflict between bpf's [2]
and bpf-next's [3]. This check was removed in bpf-next tree to allow extra
ctx_in parameter passed for XDP test runs. Reinstating the check breaks
bpf_prog_test_run_xdp logic and causes a corresponding xdp_context_test_run
selftest failure. Fix by removing the check and allow ctx_in for XDP test
runs.

  [0] 5af84df962dd ("Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net")
  [1] 947e8b595b82 ("bpf: explicitly prohibit ctx_{in, out} in non-skb BPF_PROG_TEST_RUN")
  [2] 5e21bb4e8125 ("bpf, test: fix NULL pointer dereference on invalid expected_attach_type")
  [3] 47316f4a3053 ("bpf: Support input xdp_md context in BPF_PROG_TEST_RUN")

Fixes: 5af84df962dd ("Merge git://git.kernel.org/pub/scm/linux/kernel/git/netdev/net")
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>