ceph: use vmf_error() in ceph_filemap_fault()

This code is converted to use vmf_error().

Signed-off-by: Souptick Joarder <jrdr.linux@gmail.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>