btrfs: use nodesize everywhere, kill leafsize

The nodesize and leafsize were never of different values. Unify the
usage and make nodesize the one. Cleanup the redundant checks and
helpers.

Shaves a few bytes from .text:

  text    data     bss     dec     hex filename
852418   24560   23112  900090   dbbfa btrfs.ko.before
851074   24584   23112  898770   db6d2 btrfs.ko.after

Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>
diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 2f5000c..7b6d142 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -2157,7 +2157,7 @@
 
 		bytenr = btrfs_node_blockptr(cur, path->slots[*level]);
 		ptr_gen = btrfs_node_ptr_generation(cur, path->slots[*level]);
-		blocksize = btrfs_level_size(root, *level - 1);
+		blocksize = root->nodesize;
 
 		parent = path->nodes[*level];
 		root_owner = btrfs_header_owner(parent);