s390/bpf: Describe the frame using a struct instead of constants

s390/bpf: Describe the frame using a struct instead of constants

Currently the caller-allocated portion of the stack frame is described
using constants, hardcoded values, and an ASCII drawing, making it
harder than necessary to ensure that everything is in sync.

Declare a struct and use offsetof() and offsetofend() macros to refer
to various values stored within the frame.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Link: https://lore.kernel.org/r/20250624121501.50536-3-iii@linux.ibm.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>