selftests/bpf: Fix casting error when cross-compiling test_verifier for 32-bit platforms

When cross-compiling test_verifier for 32-bit platforms, the casting error is shown below:

test_verifier.c:1263:27: error: cast from pointer to integer of different size [-Werror=pointer-to-int-cast]
 1263 |  info.xlated_prog_insns = (__u64)*buf;
      |                           ^
cc1: all warnings being treated as errors

Fix it by adding zero-extension for it.

Fixes: 933ff531

 ("selftests/bpf: specify expected instructions in test_verifier tests")
Signed-off-by: Pu Lehui <pulehui@huawei.com>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221108121945.4104644-1-pulehui@huaweicloud.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>