ext4: insert 3.18 version of fs/ext4, fs/jbd2 and associated header files

This will cause ext3 and gfs2 to not compile correctly, but allows us
to get a modern version of ext4 into 3.10.  This makes it easier to
backport newer features such as ext4 encryption into a downrev kernel.
It also fixes a number of xfstest failures that were fixed since 3.10.

The subsequent commits will fix up the 3.18 ext4 codebase so it will
compile against 3.10.

Change-Id: I9e3d96227cc7b7aa44f7b69a368771bba17ccd56
Signed-off-by: Theodore Ts'o <tytso@google.com>