bpf, test_run: Fix packet size check for live packet mode

The live packet mode uses some extra space at the start of each page to
cache data structures so they don't have to be rebuilt at every repetition.
This space wasn't correctly accounted for in the size checking of the
arguments supplied to userspace. In addition, the definition of the frame
size should include the size of the skb_shared_info (as there is other
logic that subtracts the size of this).

Together, these mistakes resulted in userspace being able to trip the
XDP_WARN() in xdp_update_frame_from_buff(), which syzbot discovered in
short order. Fix this by changing the frame size define and adding the
extra headroom to the bpf_prog_test_run_xdp() function. Also drop the
max_len parameter to the page_pool init, since this is related to DMA which
is not used for the page pool instance in PROG_TEST_RUN.

Fixes: b530e9e1

 ("bpf: Add "live packet" mode for XDP in BPF_PROG_RUN")
Reported-by:  <syzbot+0e91362d99386dc5de99@syzkaller.appspotmail.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20220310225621.53374-1-toke@redhat.com