s390/bug: Skip __WARN_trap() in call traces

s390/bug: Skip __WARN_trap() in call traces

In order to avoid rather pointless warning disassemblies of __WARN_trap()
set the PSW address to the return address of the function which called
__WARN_trap(). This is the address to which __WARN_trap() would return
in any case.

The result is a disassembly of the function which called __WARN_trap(),
which is much more helpful.

Before:

WARNING: arch/s390/kernel/setup.c:1017 at foobar+0x2c/0x20, CPU#0: swapper/0/0
...
Krnl PSW : 0704c00180000000 000003ffe0f675f4 (__WARN_trap+0x4/0x10)
...
Krnl Code: 000003ffe0f675ec: 0707                bcr     0,%r7
           000003ffe0f675ee: 0707                bcr     0,%r7
          *000003ffe0f675f0: af000001            mc      1,0
          >000003ffe0f675f4: 07fe                bcr     15,%r14
           000003ffe0f675f6: 47000700            bc      0,1792
           000003ffe0f675fa: 0707                bcr     0,%r7
           000003ffe0f675fc: 0707                bcr     0,%r7
           000003ffe0f675fe: 0707                bcr     0,%r7
Call Trace:
 [<000003ffe0f675f4>] __WARN_trap+0x4/0x10
 [<000003ffe185bc2e>] arch_cpu_finalize_init+0x26/0x60
 [<000003ffe185654c>] start_kernel+0x53c/0x5d8
 [<000003ffe010002e>] startup_continue+0x2e/0x40

Afterwards:

WARNING: arch/s390/kernel/setup.c:1017 at foobar+0x12/0x30, CPU#0: swapper/0/0
...
Krnl PSW : 0704c00180000000 000003ffe185bc2e (arch_cpu_finalize_init+0x26/0x60)
...
Krnl Code: 000003ffe185bc1c: e3f0ff98ff71        lay     %r15,-104(%r15)
           000003ffe185bc22: e3e0f0980024        stg     %r14,152(%r15)
          *000003ffe185bc28: c0e5ff45ed94        brasl   %r14,000003ffe0119750
          >000003ffe185bc2e: c0e5ffa052b9        brasl   %r14,000003ffe0c661a0
           000003ffe185bc34: c020fffe86d6        larl    %r2,000003ffe182c9e0
           000003ffe185bc3a: e548f0a80006        mvghi   168(%r15),6
           000003ffe185bc40: e548f0a00005        mvghi   160(%r15),5
           000003ffe185bc46: a7690004            lghi    %r6,4
Call Trace:
 [<000003ffe185bc2e>] arch_cpu_finalize_init+0x26/0x60
 [<000003ffe185654c>] start_kernel+0x53c/0x5d8
 [<000003ffe010002e>] startup_continue+0x2e/0x40

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>