netfs, fscache: Move /proc/fs/fscache to /proc/fs/netfs and put in a symlink

Rename /proc/fs/fscache to "netfs" and make a symlink from fscache to that.

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: Christian Brauner <christian@brauner.io>
cc: linux-fsdevel@vger.kernel.org
cc: linux-cachefs@redhat.com