LU-1205 tests: sanityn test_18 sometimes takes long time to run

LU-1205 tests: sanityn test_18 sometimes takes long time to run

This is a live-lock problem where two processes are writing to the
same mmaped file via two nodes. To write a mmap region, both processes
will do:

  acquire cl_lock -> read page -> release cl_lock-> install page.

During the above steps, the page can be truncated after the lock is
released and then immediately cancelled by the other process, so
kernel has to do page fault again and never complete.

Lustre can't handle this case well so this test case is disabled.

Signed-off-by: Jinshan Xiong <jinshan.xiong@whamcloud.com>
Signed-off-by: Bob Glossman <bogl@whamcloud.com>
Change-Id: I0cbb00a1ca68715a0b97ce369a18c53fa8de19cb
Reviewed-on: http://review.whamcloud.com/2723
Tested-by: Hudson
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>