xfstest: add defragmentation stress tests for ext4

xfstest: add defragmentation stress tests for ext4

Perform various regression tests for ext4defrag subsystem

 301'th Test1: Defragment file while other task does direct AIO
 302'th Test2: Perform defragmentation on file under buffered AIO
 	while third task does direct AIO to donor file
 303'th Test3: Two defrag tasks use common donor file.
 304'th Test4: Stress defragmentation. Several threads perform
 	fragmentation at random position use inplace=1 will
	allocate and free blocks inside defrag event improve
	load pressure.

This tests are considered dangerous because 302'th and 303'th are known
to trigger OOPS on recent kernels see:https://gist.github.com/dmonakhov/4770294

Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
Reviewed-by: Rich Johnston <rjohnston@sgi.com>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>