librbd: disable zero-copy writes by default

librbd: disable zero-copy writes by default

Add a new configuration option to allow a client to opt-in to zero-copy
writes if they can guarantee that the provided buffer will remain
stable while the IO is in-flight.

For example, QEMU cannot guarantee stable writes since it DMAs guest OS
memory through to a librbd AIO write request if not using a bounce-buffer,
and the guest OS is free to continue to modify the affected page(s) since
it will remain flagged as dirty and will eventually re-issue another write.
However, the CRC computed by the Ceph messenger layer will detect the
change and it will result in an endless session reset loop.

Fixes: https://tracker.ceph.com/issues/42889
Signed-off-by: Jason Dillaman <dillaman@redhat.com>