LU-684 tests: replace dev_read_only patch with dm-flakey

LU-684 tests: replace dev_read_only patch with dm-flakey

The dev_read_only kernel patch is mainly used for testing,
in order to simulate a server crash for ldiskfs by discarding
all of the writes to the device.

Since Linux kernel 3.0, this testing functionality can be
simulated by using "dm-flakey" target for device-mapper,
which supports a "drop_writes" parameter that could be used
in place of our dev_read_only kernel patch.

Signed-off-by: Hongchao Zhang <hongchao.zhang@intel.com>
Signed-off-by: James Simmons <uja.ornl@yahoo.com>
Signed-off-by: Jian Yu <jian.yu@intel.com>
Change-Id: I51ff9a1a10fb5bacdc1afa2716b769b5eda41863
Reviewed-on: https://review.whamcloud.com/7200
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>