osdc/Objecter: only post_rx_buffer if no op timeout

If we post an rx buffer and there is a timeout, the revocation can happen
while the reader has consumed the buffers but before it has decoded and
constructed the message.  In particular, we calculate a crc32c over the
data portion of the message after we've taken the buffers and dropped the
lock.

Instead of fixing this race (for example, by reverifying rx_buffers under
the lock while calculating the crc.. bleh), just skip the rx buffer
optimization entirely when a timeout is present.

Note that this doesn't cover the op_cancel() paths, but none of those users
provide static buffers to read into.

Fixes: #9582
Backport: firefly, dumpling
Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2598 from ceph/wip-9582

librados: fix other timeout segfault

Reviewed-by: Greg Farnum <greg@inktank.com>