Decrement the obj_request_count value when deleting an object
request from its image request's list.  Rearrange a few lines
in the surrounding code.

This resolves:
    http://tracker.ceph.com/issues/3940Signed-off-by: NAlex Elder <elder@inktank.com>
Reviewed-by: NJosh Durgin <josh.durgin@inktank.com>