samples/bpf: change _kern suffix to .bpf with BPF test programs

This commit changes the _kern suffix to .bpf with the BPF test programs.
With this modification, test programs will inherit the benefit of the
new CLANG-BPF compile target.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Link: https://lore.kernel.org/r/20230115071613.125791-11-danieltimlee@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>