s390/unwind: use current_frame_address() to unwind current task

current_stack_pointer() simply returns current value of %r15. If
current_stack_pointer() caller allocates stack (which is the case in
unwind code) %r15 points to a stack frame allocated for callees, meaning
current_stack_pointer() caller (e.g. stack_trace_save) will end up in
the stacktrace. This is not expected by stack_trace_save*() callers and
causes problems.

current_frame_address() on the other hand returns function stack frame
address, which matches %r15 upon function invocation. Using it in
get_stack_pointer() makes it more aligned with x86 implementation
(according to BACKTRACE_SELF_TEST output) and meets stack_trace_save*()
caller's expectations, notably KCSAN.

Also make sure unwind_start is always inlined.

Reported-by: Nathan Chancellor <nathan@kernel.org>
Suggested-by: Marco Elver <elver@google.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Tested-by: Marco Elver <elver@google.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>
Link: https://lore.kernel.org/r/patch.git-04dd26be3043.your-ad-here.call-01630504868-ext-6188@work.hours


Signed-off-by: Heiko Carstens <hca@linux.ibm.com>