netfs: Fix read-retry for fs with no ->prepare_read()

Fix netfslib's read-retry to only call ->prepare_read() in the backing
filesystem such a function is provided.  We can get to this point if a
there's an active cache as failed reads from the cache need negotiating
with the server instead.

Fixes: ee4cdf7b ("netfs: Speed up buffered reading")
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/529329.1736261010@warthog.procyon.org.uk


cc: Jeff Layton <jlayton@kernel.org>
cc: netfs@lists.linux.dev
cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>