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/transaction.c b/fs/btrfs/transaction.c
index 977717b..e336646 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -408,7 +408,7 @@
 	if (num_items > 0 && root != root->fs_info->chunk_root) {
 		if (root->fs_info->quota_enabled &&
 		    is_fstree(root->root_key.objectid)) {
-			qgroup_reserved = num_items * root->leafsize;
+			qgroup_reserved = num_items * root->nodesize;
 			ret = btrfs_qgroup_reserve(root, qgroup_reserved);
 			if (ret)
 				return ERR_PTR(ret);