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/inode-item.c

@@ -35,7 +35,6 @@ int btrfs_insert_inode(struct btrfs_trans_handle *trans, struct btrfs_root
 
 	path = btrfs_alloc_path();
 	BUG_ON(!path);
-	btrfs_init_path(path);
 	ret = btrfs_insert_item(trans, root, &key, inode_item,
 				sizeof(*inode_item));
 	btrfs_release_path(root, path);