rbd: return obj version in __rbd_refresh_header()

Add a new parameter to __rbd_refresh_header() through which the
version of the header object is passed back to the caller.  In most
cases this isn't needed.  The main motivation is to normalize
(almost) all calls to __rbd_refresh_header() so they are all
wrapped immediately by mutex_lock()/mutex_unlock().

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