libceph: Unlock unprocessed pages in start_read() error path

Function start_read() can get an error before processing all pages.
It must not only release the remaining pages, but unlock them too.

This fixes http://tracker.newdream.net/issues/3370



Signed-off-by: David Zafman <david.zafman@inktank.com>
Reviewed-by: Alex Elder <elder@inktank.com>