nfsd4: fix nfs4 stateid leak

Processes that open and close multiple files may end up setting this
oo_last_closed_stid without freeing what was previously pointed to.
This can result in a major leak, visible for example by watching the
nfsd4_stateids line of /proc/slabinfo.
Reported-by: NCyril B. <cbay@excellency.fr>
Tested-by: NCyril B. <cbay@excellency.fr>
Cc: stable@vger.kernel.org
Signed-off-by: NJ. Bruce Fields <bfields@redhat.com>

fs/nfsd/nfs4state.c

@@ -3766,6 +3766,7 @@ nfsd4_close(struct svc_rqst *rqstp, struct nfsd4_compound_state *cstate,
 	memcpy(&close->cl_stateid, &stp->st_stid.sc_stateid, sizeof(stateid_t));
 
 	nfsd4_close_open_stateid(stp);
+	release_last_closed_stateid(oo);
 	oo->oo_last_closed_stid = stp;
 
 	if (list_empty(&oo->oo_owner.so_stateids)) {