s390/dasd: fix diag 0x250 inline assembly

git commit 1ec2772e0c3c ("s390/diag: add a statistic for diagnose
calls") added function calls to gather diagnose statistics.

In case of the dasd diag driver the function call was added between a
register asm statement which initialized register r2 and the inline
assembly itself.  The function call clobbers the contents of register
r2 and therefore the diag 0x250 call behaves in a more or less random
way.

Fix this by extracting the function call into a separate function like
we do everywhere else.

Fixes: 1ec2772e0c3c ("s390/diag: add a statistic for diagnose calls")
Cc: <stable@vger.kernel.org> # 4.4+
Reported-and-tested-by: Stefan Haberland <sth@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

drivers/s390/block/dasd_diag.c

@@ -67,7 +67,7 @@ static const u8 DASD_DIAG_CMS1[] = { 0xc3, 0xd4, 0xe2, 0xf1 };/* EBCDIC CMS1 */
  * and function code cmd.
  * In case of an exception return 3. Otherwise return result of bitwise OR of
  * resulting condition code and DIAG return code. */
-static inline int dia250(void *iob, int cmd)
+static inline int __dia250(void *iob, int cmd)
 {
 	register unsigned long reg2 asm ("2") = (unsigned long) iob;
 	typedef union {
@@ -77,7 +77,6 @@ static inline int dia250(void *iob, int cmd)
 	int rc;
 
 	rc = 3;
-	diag_stat_inc(DIAG_STAT_X250);
 	asm volatile(
 		"	diag	2,%2,0x250\n"
 		"0:	ipm	%0\n"
@@ -91,6 +90,12 @@ static inline int dia250(void *iob, int cmd)
 	return rc;
 }
 
+static inline int dia250(void *iob, int cmd)
+{
+	diag_stat_inc(DIAG_STAT_X250);
+	return __dia250(iob, cmd);
+}
+
 /* Initialize block I/O to DIAG device using the specified blocksize and
  * block offset. On success, return zero and set end_block to contain the
  * number of blocks on the device minus the specified offset. Return non-zero