Initial code is from https://sourceforge.net/projects/tier/files/btier-2.0.1,
just make it compile on kernel 4.4.54.

BTIER 2.x differs from BTIER 1.X in several ways.
The most significant change is that BTIER 2.x no longer directly 
supports the use of files as backend devices.
The VFS interface has been removed to allow much increased performance.

BTIER 2.x is able to handle up to a million IOPS, provided that the 
backend devices can handle that workload.
BTIER 1.X is able to handle 250K writes and 100K random reads.
So the performance gain is significant.

Note that using files as backend storage is still possible with btier-2.x
by converting the files to block devices using the kernel loop driver.

IMPORTANT: BTIER-2.X works / is supported with a very recent kernel!!!
Use at least 3.14 or higher.