rbd: update feature bits

There is a new rbd feature bit defined for "fancy striping." Add
it to the ones defined in the kernel client.

Change RBD_FEATURES_ALL so it represents the set of all feature
bits (rather than just the ones we support).  Define a new symbol
RBD_FEATURES_SUPPORTED to indicate the supported ones.

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