generic: Add sudden shutdown tests for multi block atomic writes

generic: Add sudden shutdown tests for multi block atomic writes

This test is intended to ensure that multi blocks atomic writes
maintain atomic guarantees across sudden FS shutdowns.

The way we work is that we lay out a file with random mix of written,
unwritten and hole extents. Then we start performing atomic writes
sequentially on the file while we parallelly shutdown the FS. Then we
note the last offset where the atomic write happened just before shut
down and then make sure blocks around it either have completely old
data or completely new data, ie the write was not torn during shutdown.

We repeat the same with completely written, completely unwritten and completely
empty file to ensure these cases are not torn either.  Finally, we have a
similar test for append atomic writes

Suggested-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>