selftests: bpf: fix pkt grow tests

selftests: bpf: fix pkt grow tests

Skip tail adjust tests in xskxceiver for SKB mode as it is not very
friendly for it. multi-buffer case does not work as xdp_rxq_info that is
registered for generic XDP does not report ::frag_size. The non-mbuf
path copies packet via skb_pp_cow_data() which only accounts for
headroom, leaving us with no tailroom and causing underlying XDP prog to
drop packets therefore.

For multi-buffer test on other modes, change the amount of bytes we use
for growth, assume worst-case scenario and take care of headroom and
tailroom.

Reviewed-by: Björn Töpel <bjorn@kernel.org>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Link: https://patch.msgid.link/20260402154958.562179-7-maciej.fijalkowski@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>