xfstests: add disk failure simulation test

There are many situations where disk may fail for example
1) brutal usb dongle unplug
2) iscsi (or any other netbdev) failure due to network issues
In this situation filesystem which use this blockdevice is
expected to fail(force RO remount, abort, etc) but whole system
should still be operational. In other words:
1) Kernel should not panic
2) Memory should not leak
3) Data integrity operations (sync,fsync,fdatasync, directio) should fail
   for affected filesystem 
4) It should be possible to umount broken filesystem

Later when disk becomes available again we expect(only for journaled filesystems):
5) It will be possible to mount filesystem w/o explicit fsck (in order to caught
   issues like https://patchwork.kernel.org/patch/1983981/)
6) Filesystem should be operational
7) After mount/umount has being done all errors should be fixed so fsck should
   not spot any issues.

This test use fault enjection (CONFIG_FAIL_MAKE_REQUEST=y config option )
which force all new IO requests to fail for a given device. Xfs already has
XFS_IOC_GOINGDOWN ioctl which provides similar behaviour, but it is fsspeciffic
and it does it in an easy way because it perform freeze_bdev() before actual
shotdown.

Test run fsstress in background and then force disk failure.
Once disk failed it check that (1)-(4) is true.
Then makes disk available again and check that (5)-(7) is also true

BE CAREFUL!! test known to cause memory corruption for XFS
see: https://gist.github.com/dmonakhov/4945005