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>

fs/btrfs/compression.h

@@ -162,6 +162,10 @@ struct btrfs_compress_op {
 	 * if the level is out of bounds or the default if 0 is passed in.
 	 */
 	unsigned int (*set_level)(unsigned int level);
+
+	/* Maximum level supported by the compression algorithm */
+	unsigned int max_level;
+	unsigned int default_level;
 };
 
 /* The heuristic workspaces are managed via the 0th workspace manager */

fs/btrfs/lzo.c

@@ -523,4 +523,6 @@ const struct btrfs_compress_op btrfs_lzo_compress = {
 	.decompress_bio		= lzo_decompress_bio,
 	.decompress		= lzo_decompress,
 	.set_level		= lzo_set_level,
+	.max_level		= 1,
+	.default_level		= 1,
 };

fs/btrfs/zlib.c

@@ -437,4 +437,6 @@ const struct btrfs_compress_op btrfs_zlib_compress = {
 	.decompress_bio		= zlib_decompress_bio,
 	.decompress		= zlib_decompress,
 	.set_level              = zlib_set_level,
+	.max_level		= 9,
+	.default_level		= BTRFS_ZLIB_DEFAULT_LEVEL,
 };

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,
 };