btrfs: Replace opencoded sizes with their symbolic constants

Currently btrfs' code uses a mix of opencoded sizes and defines from sizes.h.
Let's unifiy the code base to always use the symbolic constants. No functional
changes

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 880ab49..4fb7eef 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2126,7 +2126,7 @@ static int btrfs_statfs(struct dentry *dentry, struct kstatfs *buf)
 	 * succeed even if the Avail is zero. But this is better than the other
 	 * way around.
 	 */
-	thresh = 4 * 1024 * 1024;
+	thresh = SZ_4M;
 
 	if (!mixed && total_free_meta - thresh < block_rsv->size)
 		buf->f_bavail = 0;