x86_64: fix the check in stack_overflow_check

Impact: make stack overflow debug check and printout narrower

stack_overflow_check() should consider the stack usage of pt_regs, and
thus it could warn us in advance. Additionally, it looks better for
the warning time to start at INITIAL_JIFFIES.

Assuming that rsp gets close to the check point before interrupt
arrives: when interrupt really happens, thread_info will be partly
overrode.

Signed-off-by: jia zhang <jia.zhang2008@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>