bpf: Refactor process_dynptr_func

This change cleans up process_dynptr_func's flow to be more intuitive
and updates some comments with more context.

Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Link: https://lore.kernel.org/r/20230301154953.641654-3-joannelkoong@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>