NFSv4/flexfiles: Update layout stats & error paths for striped layouts

Updates the layout stats logic to be stripe aware. Read and write
stats are accumulated on a per DS stripe basis. Also updates error
paths to use dss_id where appropraite.

Limitations:

1. The layout stats structure is still statically sized to 4 and there
is no deduplication logic for deviceids that may appear more than once
in a striped layout.

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

Author: Jonathan Curley