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/backref.c b/fs/btrfs/backref.c
index cfe8566..4de9792 100644
--- a/fs/btrfs/backref.c
+++ b/fs/btrfs/backref.c
@@ -482,7 +482,7 @@
 			continue;
 		BUG_ON(!ref->wanted_disk_byte);
 		eb = read_tree_block(fs_info->tree_root, ref->wanted_disk_byte,
-				     fs_info->tree_root->leafsize, 0);
+				     fs_info->tree_root->nodesize, 0);
 		if (!eb || !extent_buffer_uptodate(eb)) {
 			free_extent_buffer(eb);
 			return -EIO;
@@ -991,8 +991,8 @@
 			    ref->level == 0) {
 				u32 bsz;
 				struct extent_buffer *eb;
-				bsz = btrfs_level_size(fs_info->extent_root,
-							ref->level);
+
+				bsz = fs_info->extent_root->nodesize;
 				eb = read_tree_block(fs_info->extent_root,
 							   ref->parent, bsz, 0);
 				if (!eb || !extent_buffer_uptodate(eb)) {
@@ -1366,7 +1366,7 @@
 	}
 	btrfs_item_key_to_cpu(path->nodes[0], found_key, path->slots[0]);
 	if (found_key->type == BTRFS_METADATA_ITEM_KEY)
-		size = fs_info->extent_root->leafsize;
+		size = fs_info->extent_root->nodesize;
 	else if (found_key->type == BTRFS_EXTENT_ITEM_KEY)
 		size = found_key->offset;