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/[email protected]
Cc: Andrew Morton <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Brian Gerst <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: H. Peter Anvin <[email protected]>