x86/alternative: Report missing return thunk details

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

Suggested-by: Peter Zijlstra <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Signed-off-by: Peter Zijlstra (Intel) <[e-mail]>
Link: https://lore.kernel.org/lkml/[e-mail]/

arch/x86/kernel/alternative.c

···
 			dest = addr + insn.length + insn.immediate.value;
 
 		if (__static_call_fixup(addr, op, dest) ||
-		    WARN_ON_ONCE(dest != &__x86_return_thunk))
+		    WARN_ONCE(dest != &__x86_return_thunk,
+			      "missing return thunk: %pS-%pS: %*ph",
+			      addr, dest, 5, addr))
 			continue;
 
 		DPRINTK("return thunk at: %pS (%px) len: %d to: %pS",