btrfs: define compression levels statically

The maximum and default levels do not change and can be defined
directly. The set_level callback was a temporary solution and will be
removed.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/zstd.c b/fs/btrfs/zstd.c
index 3837ca18..b2b23a6 100644
--- a/fs/btrfs/zstd.c
+++ b/fs/btrfs/zstd.c
@@ -729,4 +729,6 @@ const struct btrfs_compress_op btrfs_zstd_compress = {
 	.decompress_bio = zstd_decompress_bio,
 	.decompress = zstd_decompress,
 	.set_level = zstd_set_level,
+	.max_level	= ZSTD_BTRFS_MAX_LEVEL,
+	.default_level	= ZSTD_BTRFS_DEFAULT_LEVEL,
 };