Merge branch 'bpf-allow-utf-8-literals-in-bpf_bprintf_prepare'

Yihan Ding says:

====================
bpf: allow UTF-8 literals in bpf_bprintf_prepare()

bpf_bprintf_prepare() currently rejects any non-ASCII byte in format
strings, so helpers such as bpf_trace_printk() fail to emit UTF-8
literal text even when those bytes are not part of a format specifier.

Keep plain text permissive while continuing to parse '%' sequences as
ASCII-only. Patch 1 updates snprintf_negative() at the same time so the
selftests stay consistent during bisection. Patch 2 then extends
trace_printk coverage for both the valid UTF-8 literal case and the
invalid non-ASCII-after-'%' case.

Changes in v3:
- drop Suggested-by trailers and move review credit into this changelog
- update test_snprintf_negative() in patch 1/2 so plain non-ASCII text is
  accepted while non-ASCII after '%' is still rejected, keeping
  ./test_progs -t snprintf aligned with the new behavior.
- clarify the trace_printk negative case with an explicit invalid format
  string and comment
- address Paul Chaignon's review feedback and keep the negative coverage
  requested earlier by Alan Maguire

Changes in v2:
- split the core change and selftest updates into two patches
- drop unnecessary isspace()/ispunct() casts
- add comments to clarify plain-text vs format-specifier handling
- add a negative selftest for non-ASCII bytes inside '%' sequences

Testing:
- Reproduced on x86_64 without the core fix: ASCII trace output works,
  while UTF-8 literal text in bpf_trace_printk() is rejected and
  produces no trace output
- Verified with tools/testing/selftests/bpf: ./test_progs -t trace_printk
- Verified with tools/testing/selftests/bpf: ./test_progs -t snprintf
====================

Link: https://patch.msgid.link/20260416120142.1420646-1-dingyihan@uniontech.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>