uprobes: revert ref_ctr_offset in uprobe_unregister error path

uprobes: revert ref_ctr_offset in uprobe_unregister error path

There's error path that could lead to inactive uprobe:

  1) uprobe_register succeeds - updates instruction to int3 and
     changes ref_ctr from 0 to 1
  2) uprobe_unregister fails  - int3 stays in place, but ref_ctr
     is changed to 0 (it's not restored to 1 in the fail path)
     uprobe is leaked
  3) another uprobe_register comes and re-uses the leaked uprobe
     and succeds - but int3 is already in place, so ref_ctr update
     is skipped and it stays 0 - uprobe CAN NOT be triggered now
  4) uprobe_unregister fails because ref_ctr value is unexpected

Fix this by reverting the updated ref_ctr value back to 1 in step 2),
which is the case when uprobe_unregister fails (int3 stays in place), but
we have already updated refctr.

The new scenario will go as follows:

  1) uprobe_register succeeds - updates instruction to int3 and
     changes ref_ctr from 0 to 1
  2) uprobe_unregister fails  - int3 stays in place and ref_ctr
     is reverted to 1..  uprobe is leaked
  3) another uprobe_register comes and re-uses the leaked uprobe
     and succeds - but int3 is already in place, so ref_ctr update
     is skipped and it stays 1 - uprobe CAN be triggered now
  4) uprobe_unregister succeeds

Link: https://lkml.kernel.org/r/20250514101809.2010193-1-jolsa@kernel.org
Fixes: 1cc33161a83d ("uprobes: Support SDT markers having reference count (semaphore)")
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: David Hildenbrand <david@redhat.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Suggested-by: Oleg Nesterov <oleg@redhat.com>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>