NFSd: call rpc_destroy_wait_queue() from free_client()

commit 4cb57e3032d4e4bf5e97780e9907da7282b02b0c upstream.

Mainly to ensure that we don't leave any hanging timers.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>