netfs: Make ops->init_rreq() optional

Make the ops->init_rreq() callback optional.  This isn't required for the
erofs changes I'm implementing to do on-demand read through fscache[1].
Further, ceph has an empty init_rreq method that can then be removed and
it's marked optional in the documentation.

Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/20211227125444.21187-1-jefflexu@linux.alibaba.com/ [1]
Link: https://lore.kernel.org/r/20211228124419.103020-1-jefflexu@linux.alibaba.com
Link: https://lore.kernel.org/r/164251410387.3435901.2504600788262093313.stgit@warthog.procyon.org.uk/ # v1