rbd: there is really only one op

rbd: there is really only one op

Throughout the rbd code there are spots where it appears we can
handle an osd request containing more than one osd request op.

But that is only the way it appears.  In fact, currently only one
operation at a time can be supported, and supporting more than
one will require much more than fleshing out the support that's
there now.

This patch changes names to make it perfectly clear that anywhere
we're dealing with a block of ops, we're in fact dealing with
exactly one of them.  We'll be able to simplify some things as
a result.

When multiple op support is implemented, we can update things again
accordingly.

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