x86: Make save_stack_address() !CONFIG_FRAME_POINTER friendly

If CONFIG_FRAME_POINTER=n, print_context_stack() shouldn't neglect the
non-reliable addresses on stack, this is all we have if dump_trace(bp)
is called with the wrong or zero bp.

For example, /proc/pid/stack doesn't work if CONFIG_FRAME_POINTER=n.

This patch obviously has no effect if CONFIG_FRAME_POINTER=y, otherwise
it reverts 1650743c "x86: don't save unreliable stack trace entries".

Also, remove the unnecessary type-cast.

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>
Cc: Andrew Morton <akpm@linux-foundation.org>
LKML-Reference: <20100603193239.GA31530@redhat.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>