rbd: drop flags parameter from rbd_req_sync_exec()

All callers of rbd_req_sync_exec() pass CEPH_OSD_FLAG_READ as their
flags argument.  Delete that parameter and use CEPH_OSD_FLAG_READ
within the function.  If we find a need to support write operations
we can add it back again.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>