block: fix stacking of atomic writes when atomics are not supported

block: fix stacking of atomic writes when atomics are not supported

Atomic writes support may not always be possible when stacking devices
which support atomic writes. Such as case is a different atomic write
boundary between stacked devices (which is not supported).

In the case that atomic writes cannot supported, the top device queue HW
limits are set to 0.

However, in blk_stack_atomic_writes_limits(), we detect that we are
stacking the first bottom device by checking the top device
atomic_write_hw_max value == 0. This get confused with the case of atomic
writes not supported, above.

Make the distinction between stacking the first bottom device and no
atomics supported by initializing stacked device atomic_write_hw_max =
UINT_MAX and checking that for stacking the first bottom device.

Fixes: d7f36dc446e8 ("block: Support atomic writes limits for stacked devices")
Signed-off-by: John Garry <john.g.garry@oracle.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>