powerpc/livepatch: return -ERRNO values in save_stack_trace_tsk_reliable()

To match its x86 counterpart, save_stack_trace_tsk_reliable() should
return -EINVAL in cases that it is currently returning 1.  No caller is
currently differentiating non-zero error codes, but let's keep the
arch-specific implementations consistent.

Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>