ext4: cleanup ext4_ext_grow_indepth code

Currently code make an impression what grow procedure is very complicated
and some mythical paths, blocks are involved. But in fact grow in depth
it relatively simple procedure:
 1) Just create new meta block and copy root data to that block.
 2) Convert root from extent to index if old depth == 0
 3) Update root block pointer

This patch does:
 - Reorganize code to make it more self explanatory
 - Do not pass path parameter to new_meta_block() in order to
   provoke allocation from inode's group because top-level block
   should site closer to it's inode, but not to leaf data block.

   [ This happens anyway, due to logic in mballoc; we should drop
     the path parameter from new_meta_block() entirely.  -- tytso ]

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>