generic: various atomic write tests with hardware and scsi_debug

generic: various atomic write tests with hardware and scsi_debug

Simple tests of various atomic write requests and a (simulated) hardware
device.

The first test performs basic multi-block atomic writes on a scsi_debug device
with atomic writes enabled. We test all advertised sizes between the atomic
write unit min and max. We also ensure that the write fails when expected, such
as when attempting buffered io or unaligned directio.

The second test is similar to the one above, except that it verifies multi-block
atomic writes on actual hardware instead of simulated hardware. The device used
in this test is not required to support atomic writes.

The final two tests ensure multi-block atomic writes can be performed on various
interweaved mappings, including written, mapped, hole, and unwritten. We also
test large atomic writes on a heavily fragmented filesystem. These tests are
separated into reflink (shared) and non-reflink tests.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Catherine Hoang <catherine.hoang@oracle.com>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Ojaswin Mujoo <ojaswin@linux.ibm.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>