samples: bpf: Add redirect tracepoint statistics support

This implements per-errno reporting (for the ones we explicitly
recognize), adds some help output, and implements the stats retrieval
and printing functions.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210821002010.845777-6-memxor@gmail.com