zram: failing to decompress is WARN_ON worthy

If we fail to decompress in zram it's a pretty serious problem.  We were
entrusted to be able to decompress the old data but we failed.  Either
we've got some crazy bug in the compression code or we've got memory
corruption.

At the moment, when this happens the log looks like this:

  ERR kernel: [ 1833.099861] zram: Decompression failed! err=-22, page=336112
  ERR kernel: [ 1833.099881] zram: Decompression failed! err=-22, page=336112
  ALERT kernel: [ 1833.099886] Read-error on swap-device (253:0:2688896)

It is true that we have an "ALERT" level log in there, but (at least to
me) it feels like even this isn't enough to impart the seriousness of this
error.  Let's convert to a WARN_ON.  Note that WARN_ON is automatically
"unlikely" so we can simply replace the old annotation with the new one.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Sonny Rao <sonnyrao@chromium.org>
Cc: Jens Axboe <axboe@kernel.dk>
Link: https://lkml.kernel.org/r/20200917174059.1.If09c882545dbe432268f7a67a4d4cfcb6caace4f@changeidSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/block/zram/zram_drv.c

@@ -1270,7 +1270,7 @@ static int __zram_bvec_read(struct zram *zram, struct page *page, u32 index,
 	zram_slot_unlock(zram, index);
 
 	/* Should NEVER happen. Return bio error if it does. */
-	if (unlikely(ret))
+	if (WARN_ON(ret))
 		pr_err("Decompression failed! err=%d, page=%u\n", ret, index);
 
 	return ret;