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>
Change-Id: I0c3fd5beab388cbed6193ae496d4d0e4f23be797
Reviewed-on: http://review.whamcloud.com/2544
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>