osdc/Objecter: revoke rx_buffer on op_cancel

If we cancel a read, revoke the rx buffers to avoid a use-after-free and/or
other undefined badness by using user buffers that may no longer be
present.

Fixes: #9362
Backport: firefly, dumpling
Reported-by: Matthias Kiefer <matthias.kiefer@1und1.de>
Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit 2305b2897acba38384358c33ca3bbfcae6f1c74e)

(adjusted for op->con instead of s->con)