x86/dumpstack: Add show_stack_regs() and use it

Add a helper to dump supplied pt_regs and use it in the MSR exception
handling code to have precise stack traces pointing to the actual
function causing the MSR access exception and not the stack frame of the
exception handler itself.

The new output looks like this:

 unchecked MSR access error: RDMSR from 0xdeadbeef at rIP: 0xffffffff8102ddb6 (early_init_intel+0x16/0x3a0)
  00000000756e6547 ffffffff81c03f68 ffffffff81dd0940 ffffffff81c03f10
  ffffffff81d42e65 0000000001000000 ffffffff81c03f58 ffffffff81d3e5a3
  0000800000000000 ffffffff81800080 ffffffffffffffff 0000000000000000
 Call Trace:
  [<ffffffff81d42e65>] early_cpu_init+0xe7/0x136
  [<ffffffff81d3e5a3>] setup_arch+0xa5/0x9df
  [<ffffffff81d38bb9>] start_kernel+0x9f/0x43a
  [<ffffffff81d38294>] x86_64_start_reservations+0x2f/0x31
  [<ffffffff81d383fe>] x86_64_start_kernel+0x168/0x176

Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1467671487-10344-4-git-send-email-bp@alien8.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>