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/20240902105656.1383858-13-senozhatsky@chromium.org


Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Nick Terrell <terrelln@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>