rbd: do a safe list traversal in rbd_img_request_submit()

It's possible that the reference to the object request dropped
inside the loop in rbd_img_request_submit() will be the last
one, in which case the content of the object pointer can't be
trusted.

Use a safe form of the object request list traversal to avoid
problems.

This resolves:
    http://tracker.ceph.com/issues/4705

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