NFSv4/flexfiles: Use ds_commit_idx when marking a write commit
Correct this path to use ds_commit_idx. Another noop preparation
change. In current code commit_idx == mirror_idx but when striping is
enabled that will not be true.
Signed-off-by: Jonathan Curley <jcurley@purestorage.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>
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