f2fs: clarify compress level bit offset

commit 3fde13f8 ("f2fs: compress: support compress level") introduce
compress level, which macro(COMPRESS_LEVEL_OFFSET) is 8, But use wrong
comment about compress level.

Let's fix it.
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

include/linux/f2fs_fs.h

@@ -315,7 +315,7 @@ struct f2fs_inode {
 			__u8 i_log_cluster_size;	/* log of cluster size */
 			__le16 i_compress_flag;		/* compress flag */
 						/* 0 bit: chksum flag
-						 * [10,15] bits: compress level
+						 * [8,15] bits: compress level
 						 */
 			__le32 i_extra_end[0];	/* for attribute size calculation */
 		} __packed;