NFS: Fix up a typo in nfs_dns_ent_put

call_rcu() needs to take a first argument of type (struct rcu_head *).

Fixes: fd497f1e40d9 ("NFS: Lockless DNS lookups")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>