stacktrace: Unbreak stack_trace_save_tsk_reliable()

Miroslav reported that the livepatch self-tests were failing, specifically
a case in which the consistency model ensures that a current executing
function is not allowed to be patched, "TEST: busy target module".

Recent renovations of stack_trace_save_tsk_reliable() left it returning
only an -ERRNO success indication in some configuration combinations:

  klp_check_stack()
    ret = stack_trace_save_tsk_reliable()
      #ifdef CONFIG_ARCH_STACKWALK && CONFIG_HAVE_RELIABLE_STACKTRACE
        stack_trace_save_tsk_reliable()
          ret = arch_stack_walk_reliable()
            return 0
            return -EINVAL
          ...
          return ret;
    ...
    if (ret < 0)
      /* stack_trace_save_tsk_reliable error */
    nr_entries = ret;                               << 0

Previously (and currently for !CONFIG_ARCH_STACKWALK &&
CONFIG_HAVE_RELIABLE_STACKTRACE) stack_trace_save_tsk_reliable() returned
the number of entries that it consumed in the passed storage array.

In the case of the above config and trace, be sure to return the
stacktrace_cookie.len on stack_trace_save_tsk_reliable() success.

Fixes: 25e39e32b0a3f ("livepatch: Simplify stack trace retrieval")
Reported-by: Miroslav Benes <[email protected]>
Signed-off-by: Joe Lawrence <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Kamalesh Babulal <[email protected]>
Acked-by: Josh Poimboeuf <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]

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