ceph: pass down the flags to grab_cache_page_write_begin

write_begin operations are passed a flags parameter that we need to
mirror here, so that we don't (e.g.) recurse back into filesystem code
inappropriately.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>