NFSD: Clean up forgetting clients

I added in a generic for-each loop that takes a pass over the client_lru
list for the current net namespace and calls some function.  The next few
patches will update other operations to use this function as well.  A value
of 0 still means "forget everything that is found".

Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>