nfsd: add a list_head arg to nfsd_foreach_client_lock

In a later patch, we'll want to collect the locks onto a list for later
destruction. If "func" is defined and "collect" is defined, then we'll
add the lock stateid to the list.

Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>