nfsv4 client: replace while loops with LIST_FOREACH() loops

This patch replaces a couple of while() loops with LIST_FOREACH() loops.
While here, declare a couple of variables "bool".
I think LIST_FOREACH() is preferred and makes the code more readable.
This also prepares the code for future changes to use a hash table of
lists for open searches via file handle.

This patch should not result in a semantics change.

MFC after: 2 weeks