nfsd4: Fix forced-expiry locking

This should use the network-namespace-wide client_lock, not the
per-client cl_lock.

You shouldn't see any bugs unless you're actually using the
forced-expiry interface introduced by 89c905be.

Fixes: 89c905be

 "nfsd: allow forced expiration of NFSv4 clients"
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>