x86: Unify save_stack_address() and save_stack_address_nosched()

Cleanup. Factor the common code in save_stack_address() and
save_stack_address_nosched().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Roland McGrath <roland@redhat.com>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: Vegard Nossum <vegard.nossum@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
LKML-Reference: <20100603193243.GA31534@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>