selftests/bpf: Fix xdp_pull_data failure with 64K page

selftests/bpf: Fix xdp_pull_data failure with 64K page

If the argument 'pull_len' of run_test() is 'PULL_MAX' or
'PULL_MAX | PULL_PLUS_ONE', the eventual pull_len size
will close to the page size. On arm64 systems with 64K pages,
the pull_len size will be close to 64K. But the existing buffer
will be close to 9000 which is not enough to pull.

For those failed run_tests(), make buff size to
  pg_sz + (pg_sz / 2)
This way, there will be enough buffer space to pull
regardless of page size.

Tested-by: Alan Maguire <alan.maguire@oracle.com>
Cc: Amery Hung <ameryhung@gmail.com>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Amery Hung <ameryhung@gmail.com>
Link: https://lore.kernel.org/r/20260123055128.495265-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>