x86/alternative: Report missing return thunk details

x86/alternative: Report missing return thunk details

commit 65cdf0d623bedf0e069bb64ed52e8bb20105e2ba upstream.

Debugging missing return thunks is easier if we can see where they're
happening.

Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/lkml/Ys66hwtFcGbYmoiZ@hirez.programming.kicks-ass.net/
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>