nfsd4.1: fix session memory use calculation

Unbalanced calculations on creation and destruction of sessions could
cause our estimate of cache memory used to become negative, sometimes
resulting in spurious SERVERFAULT returns to client CREATE_SESSION
requests.
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/nfs4state.c

@@ -629,10 +629,13 @@ void
 free_session(struct kref *kref)
 {
 	struct nfsd4_session *ses;
+	int mem;
 
 	ses = container_of(kref, struct nfsd4_session, se_ref);
 	spin_lock(&nfsd_drc_lock);
-	nfsd_drc_mem_used -= ses->se_fchannel.maxreqs * NFSD_SLOT_CACHE_SIZE;
+	mem = ses->se_fchannel.maxreqs
+		* (ses->se_fchannel.maxresp_cached - NFSD_MIN_HDR_SEQ_SZ);
+	nfsd_drc_mem_used -= mem;
 	spin_unlock(&nfsd_drc_lock);
 	free_session_slots(ses);
 	kfree(ses);