ceph: allocate non-zero page to fscache in readpage()

ceph_osdc_readpages() returns number of bytes read, currently,
the code only allocate full-zero page into fscache, this patch
fixes this.

Signed-off-by: Li Wang <liwang@ubuntukylin.com>
Reviewed-by: Milosz Tanski <milosz@adfin.com>
Reviewed-by: Sage Weil <sage@inktank.com>

fs/ceph/addr.c

@@ -216,7 +216,7 @@ static int readpage_nounlock(struct file *filp, struct page *page)
 	}
 	SetPageUptodate(page);
 
-	if (err == 0)
+	if (err >= 0)
 		ceph_readpage_to_fscache(inode, page);
 
 out: