rbd: new request completion code

Do away with partial request completions and all the associated
complexity.  Individual object requests no longer need to be completed
in order -- when the last one becomes ready, we complete the entire
higher level request all at once.

This also wraps up the conversion to a state machine model and
eliminates the recursion described in commit 6d69bb53 ("rbd:
prevent kernel stack blow up on rbd map").

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>