netfs: add a new flag indicating that readahead should not be done

With ceph, if we don't have appropriate caps, then it's best to give up
on doing any readahead. Add a mechanism to allow its init_rreq op to
indicate that.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: David Howells <dhowells@redhat.com>