zram: check that backends array has at least one backend

zram: check that backends array has at least one backend

Make sure that backends array has anything apart from the
sentinel NULL value.

We also select LZO_BACKEND if none backends were selected.

Link: https://lkml.kernel.org/r/20240712051850.484318-13-senozhatsky@chromium.org
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Nick Terrell <terrelln@fb.com>
Cc: Nitin Gupta <ngupta@vflare.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>