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/btrfs_inode.h

@@ -19,6 +19,7 @@
 #ifndef __BTRFS_I__
 #define __BTRFS_I__
 
+/* in memory btrfs inode */
 struct btrfs_inode {
 	struct btrfs_root *root;
 	struct btrfs_block_group_cache *block_group;