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.

 N.B. it might be a good idea to introduce get_uprobe() to reflect
put_uprobe() later, but it is not a subject of this patchset.

v5:
 - add fill_page() helper
 - preallocate first slot xol_area for return probes, see xol_get_area()
   changes
 - add uretprobe_trampoline_vaddr() helper, to emphasize area->vaddr
   overload
v4:
 - get rid of area->rp_trampoline_vaddr as it always the same as ->vaddr
 - cleanup ->return_uprobes list in uprobe_free_utask(), because the
   task can exit from inside the ret-probe'd function(s).
 - in find_active_uprobe(): Once we inserted "int3" we must ensure that
   handle_swbp() will be called even if this uprobe goes away. We have
   the reference but it only protects uprobe itself, it can't protect
   agains delete_uprobe().
   IOW, we must ensure that uprobe_pre_sstep_notifier() can't return 0.
v3:
 - protected uprobe with refcounter. See atomic_inc in prepare_uretprobe()
and put_uprobe() in a following patch in handle_uretprobe()
v2:
 - get rid of ->return_consumers member from struct uprobe, introduce
rp_handler() in consumer

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.

v5:
 - rework handle_uretprobe() -IN PROGRESS-
v4:
 - check, whether utask is not NULL in handle_uretprobe()
   ? do we want a printk() for this case?
 - get rid of area->rp_trampoline_vaddr
 - minor handle_uretprobe() fixups
v3:
 - protected uprobe with refcounter. See put_uprobe() in handle_uretprobe()
that reflects increment in prepare_uretprobe()
v2:
 - get rid of ->return_consumers member from struct uprobe, introduce
rp_handler() in consumer instead

Signed-off-by: Anton Arapov <anton@redhat.com>
uretprobes: implemented, thus remove -ENOSYS

  1/6 and 6/6 patches are here to enclose return probes implementation
as well as prohibit uprobe_register() routine with no consumer set.

v3 changes: (the patch is introduced in v3)
  - remove 'TODO' as return probes implemented now

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