x86/unwind: Dump stack data on warnings

The unwinder warnings are good at finding unexpected unwinder issues,
but they often don't give enough data to be able to fully diagnose them.
Print a one-time stack dump when a warning is detected.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/15607370e3ddb1732b6a73d5c65937864df16ac8.1481904011.git.jpoimboe@redhat.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>