LU-13805 llite: make page_list_{add,del} symmetric

LU-13805 llite: make page_list_{add,del} symmetric

An earlier patch created the slightly frightening situation
where we use cl_page_list_del to remove references which
were not taken by cl_page_list_add.

This assymetry is scary, so let's not do it.  Instead, DIO
now explicitly puts the only cl_page reference it takes.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I832d8ca7dc7f2f99dc30f972197bebc83b8b5977