Merge branch 'fix-global-subprog-ptr_to_ctx-arg-handling'

Andrii Nakryiko says:

====================
Fix global subprog PTR_TO_CTX arg handling

Fix confusing and incorrect inference of PTR_TO_CTX argument type in BPF
global subprogs. For some program types (iters, tracepoint, any program type
that doesn't have fixed named "canonical" context type) when user uses (in
a correct and valid way) a pointer argument to user-defined anonymous struct
type, verifier will incorrectly assume that it has to be PTR_TO_CTX argument.
While it should be just a PTR_TO_MEM argument with allowed size calculated
from user-provided (even if anonymous) struct.

This did come up in practice and was very confusing to users, so let's prevent
this going forward. We had to do a slight refactoring of
btf_get_prog_ctx_type() to make it easy to support a special s390x KPROBE use
cases. See details in respective patches.

v1->v2:
  - special-case typedef bpf_user_pt_regs_t handling for KPROBE programs,
    fixing s390x after changes in patch #2.
====================

Link: https://lore.kernel.org/r/20240212233221.2575350-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>