rbd: introduce rbd_init_layout()

Rather than initializing layout fields with some made up values in
__rbd_dev_create(), move the initialization into rbd_init_layout() and
call it after the header is actually populated.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>