x86: Fix dumpstack_64 irq stack handling

Commit 2223f6f6 "x86: Clean up dumpstack_64.c code" changed
the irq_stack processing a little from what it was before.
The irq_stack_end variable needed to be cleared after its first
use. By setting irq_stack to the per cpu irq_stack and passing
that to analyze_stack(), and then clearing it after it is processed,
we can get back the original behavior.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>