s390/diag: unpoison diag224() output buffer

Diagnose 224 stores 4k bytes, which currently cannot be deduced from the
inline assembly constraints.  This leads to KMSAN false positives.

Fix the constraints by using a 4k-sized struct instead of a raw pointer. 
While at it, prettify them too.

Link: https://lkml.kernel.org/r/20240621113706.315500-29-iii@linux.ibm.comSigned-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Suggested-by: Heiko Carstens <hca@linux.ibm.com>
Reviewed-by: Alexander Potapenko <glider@google.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: <kasan-dev@googlegroups.com>
Cc: Marco Elver <elver@google.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Roman Gushchin <roman.gushchin@linux.dev>
Cc: Steven Rostedt (Google) <rostedt@goodmis.org>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/s390/kernel/diag.c

@@ -278,12 +278,14 @@ int diag224(void *ptr)
 	int rc = -EOPNOTSUPP;
 
 	diag_stat_inc(DIAG_STAT_X224);
-	asm volatile(
-		"	diag	%1,%2,0x224\n"
-		"0:	lhi	%0,0x0\n"
+	asm volatile("\n"
+		"	diag	%[type],%[addr],0x224\n"
+		"0:	lhi	%[rc],0\n"
 		"1:\n"
 		EX_TABLE(0b,1b)
-		: "+d" (rc) :"d" (0), "d" (addr) : "memory");
+		: [rc] "+d" (rc)
+		, "=m" (*(struct { char buf[PAGE_SIZE]; } *)ptr)
+		: [type] "d" (0), [addr] "d" (addr));
 	return rc;
 }
 EXPORT_SYMBOL(diag224);