fuse: get page reference for readpages

Acquire a page ref on pages in ->readpages() and release them when the
read has finished.  Not acquiring a reference didn't seem to cause any
trouble since the page is locked and will not be kicked out of the
page cache during the read.

However the following patches will want to remove the page from the
cache so a separate ref is needed.  Making the reference in req->pages
explicit also makes the code easier to understand.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>