async_tx: fix asynchronous raid6 recovery for ddf layouts

The raid6 recovery code currently requires special handling of the
4-disk and 5-disk recovery scenarios for the native layout.  Quoting
from commit 0a82a623:

     In these situations the default N-disk algorithm will present
     0-source or 1-source operations to dma devices.  To cover for
     dma devices where the minimum source count is 2 we implement
     4-disk and 5-disk handling in the recovery code.

The ddf layout presents disks=6 and disks=7 to the recovery code in
these situations.  Instead of looking at the number of disks count the
number of non-zero sources in the list and call the special case code
when the number of non-failed sources is 0 or 1.

[neilb@suse.de: replace 'ddf' flag with counting good sources]
Signed-off-by: Dan Williams <dan.j.williams@intel.com>