NFSv4/flexfiles: Commit path updates for striped layouts

NFSv4/flexfiles: Commit path updates for striped layouts

Updates the commit path to be stripe aware. This required updating
the ds_commit_idx to be stripe aware.

ds_commit_idx == mirror_idx * dss_count + dss_id.

Updates code paths to utilize the new ds_commit_idx and derive dss_id
& mirror_idx where appropriate to contact the correct DS using the
corresponding parameters.

Signed-off-by: Jonathan Curley <jcurley@purestorage.com>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>