ceph: remove useless page->mapping check in writepage_nounlock()

Callers of writepage_nounlock() have already ensured non-null
page->mapping.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>