s390/32bit: fix cmpxchg64

Fix broken inline assembly contraints for cmpxchg64 on 32bit.

Fixes this crash:

specification exception: 0006 [#1] SMP
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.13.0 #4
task: 005a16c8 ti: 00592000 task.ti: 00592000
Krnl PSW : 070ce000 8029abd6 (lockref_get+0x3e/0x9c)
...
Krnl Code: 8029abcc: a71a0001           ahi     %r1,1
           8029abd0: 1852               lr      %r5,%r2
          #8029abd2: bb40f064           cds     %r4,%r0,100(%r15)
          >8029abd6: 1943               cr      %r4,%r3
           8029abd8: 1815               lr      %r1,%r5
Call Trace:
([<0000000078e01870>] 0x78e01870)
 [<000000000021105a>] sysfs_mount+0xd2/0x1c8
 [<00000000001b551e>] mount_fs+0x3a/0x134
 [<00000000001ce768>] vfs_kern_mount+0x44/0x11c
 [<00000000001ce864>] kern_mount_data+0x24/0x3c
 [<00000000005cc4b8>] sysfs_init+0x74/0xd4
 [<00000000005cb5b4>] mnt_init+0xe0/0x1fc
 [<00000000005cb16a>] vfs_caches_init+0xb6/0x14c
 [<00000000005be794>] start_kernel+0x318/0x33c
 [<000000000010001c>] _stext+0x1c/0x80

Reported-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>