nfsv4 client: factor loop contents out into a separate function

Commit fdc9b2d50fe9 replaced a couple of while loops with LIST_FOREACH()
loops. This patch factors the body of that loop out into a separate
function called nfscl_checkown().
This 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