rbd: update mapping size only on refresh
rbd: update mapping size only on refresh

There is no sense in trying to update the mapping size before it's even
been set.

Signed-off-by: Ilya Dryomov <33f1a10eef0a3354755a1541edfdb100ff87fcfc@inktank.com>
Reviewed-by: Alex Elder <f429030cf5c0faf36fac3d102073b6e63a647baa@linaro.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs