selftest/bpf: Fix compilation warnings in 32-bit mode

Fix compilation warnings emitted when compiling selftests for 32-bit platform
(x86 in my case).

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200813204945.1020225-3-andriin@fb.com