ext4: rename uninitialized extents to unwritten

Currently in ext4 there is quite a mess when it comes to naming
unwritten extents. Sometimes we call it uninitialized and sometimes we
refer to it as unwritten.

The right name for the extent which has been allocated but does not
contain any written data is _unwritten_. Other file systems are
using this name consistently, even the buffer head state refers to it as
unwritten. We need to fix this confusion in ext4.

This commit changes every reference to an uninitialized extent (meaning
allocated but unwritten) to unwritten extent. This includes comments,
function names and variable names. It even covers abbreviation of the
word uninitialized (such as uninit) and some misspellings.

This commit does not change any of the code paths at all. This has been
confirmed by comparing md5sums of the assembly code of each object file
after all the function names were stripped from it.

Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>