btrfs: only enable extent map shrinker for DEBUG builds

Although there are several patches improving the extent map shrinker,
there are still reports of too frequent shrinker behavior, taking too
much CPU for the kswapd process.

So let's only enable extent shrinker for now, until we got more
comprehensive understanding and a better solution.

Link: https://lore.kernel.org/linux-btrfs/3df4acd616a07ef4d2dc6bad668701504b412ffc.camel@intelfx.name/
Link: https://lore.kernel.org/linux-btrfs/c30fd6b3-ca7a-4759-8a53-d42878bf84f7@gmail.com/
Fixes: 956a17d9 ("btrfs: add a shrinker for extent maps")
CC: stable@vger.kernel.org # 6.10+
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/super.c

@@ -2402,7 +2402,13 @@ static long btrfs_nr_cached_objects(struct super_block *sb, struct shrink_contro
 
 	trace_btrfs_extent_map_shrinker_count(fs_info, nr);
 
-	return nr;
+	/*
+	 * Only report the real number for DEBUG builds, as there are reports of
+	 * serious performance degradation caused by too frequent shrinks.
+	 */
+	if (IS_ENABLED(CONFIG_BTRFS_DEBUG))
+		return nr;
+	return 0;
 }
 
 static long btrfs_free_cached_objects(struct super_block *sb, struct shrink_control *sc)