Merge branch 'samples/bpf: fix LLVM compilation warning'

"Daniel T. Lee" says:

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Currently, compiling samples/bpf with LLVM emits several warning. They
are only small details, but they do not appear when compiled with GCC.
Detailed compilation command and warning logs can be found from bpf CI.
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Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>