x86: Clean up dumpstack_64.c code

x86: Clean up dumpstack_64.c code

The dump_trace() function in dumpstack_64.c is hard to follow.
The test for exception stack is processed differently than the
test for irq stack, and the normal stack is outside completely.

By restructuring this code to have all the stacks determined by
a single function that returns an enum of the following:

 STACK_IS_NORMAL
 STACK_IS_EXCEPTION
 STACK_IS_IRQ
 STACK_IS_UNKNOWN

and has the logic of each within a switch statement.
This should make the code much easier to read and understand.

Link: http://lkml.kernel.org/r/20110806012354.684598995@goodmis.org
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Brian Gerst <brgerst@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/20140206144322.086050042@goodmis.org
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>