uretprobes: return probe entry, prepare uretprobe

When a uprobe with return consumer is hit, prepare_uretprobe function is
invoked. It creates return_instance, hijacks return address and replaces
it with the trampoline.

v2:
  get rid of ->return_consumers

todo:
  protect uprobe

Signed-off-by: Anton Arapov <anton@redhat.com>
uretprobes: invoke return probe handlers

uretprobe handlers are invoked when the trampoline is hit, on completion
the trampoline is replaced with the saved return address and the
uretprobe instance deleted.

v2:
  get rid of ->return_consumers
  use rp_handler()

Signed-off-by: Anton Arapov <anton@redhat.com>
uretprobes: bypass logic

  todo: description.

PR #10078: http://pl.usenet.digipedia.org/thread/19373/11895/

Signed-off-by: Anton Arapov <anton@redhat.com>