btrfs: Code cleanup

Attaching below is some of the code cleanups that i came across while
reading the code.

a) alloc_path already calls init_path.
b) Mention that btrfs_inode is the in memory copy.Ext4 have ext4_inode_info as
the in memory copy ext4_inode as the disk copy

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/ctree.c

@@ -1647,7 +1647,6 @@ int btrfs_insert_item(struct btrfs_trans_handle *trans, struct btrfs_root
 
 	path = btrfs_alloc_path();
 	BUG_ON(!path);
-	btrfs_init_path(path);
 	ret = btrfs_insert_empty_item(trans, root, path, cpu_key, data_size);
 	if (!ret) {
 		ptr = btrfs_item_ptr(btrfs_buffer_leaf(path->nodes[0]),