As reported by Anatol Pomozov, io_destroy() fails to report an error when
it loses the race to destroy a given ioctx.  Since there is a difference in
behaviour between the thread that wins the race (which blocks on outstanding
io requests) versus lthe thread that loses (which returns immediately), wire
up a return code from kill_ioctx() to the io_destroy() syscall.

Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Cc: Anatol Pomozov <anatol.pomozov@gmail.com>