NFSD: Use struct_size() helper in alloc_session()

Use struct_size() helper to simplify the code, no functional changes.
Signed-off-by: NXiu Jianfeng <xiujianfeng@huawei.com>
Reviewed-by: NJeff Layton <jlayton@kernel.org>
Signed-off-by: NChuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4state.c

@@ -1833,13 +1833,12 @@ static struct nfsd4_session *alloc_session(struct nfsd4_channel_attrs *fattrs,
 	int numslots = fattrs->maxreqs;
 	int slotsize = slot_bytes(fattrs);
 	struct nfsd4_session *new;
-	int mem, i;
+	int i;
 
-	BUILD_BUG_ON(NFSD_MAX_SLOTS_PER_SESSION * sizeof(struct nfsd4_slot *)
-			+ sizeof(struct nfsd4_session) > PAGE_SIZE);
-	mem = numslots * sizeof(struct nfsd4_slot *);
+	BUILD_BUG_ON(struct_size(new, se_slots, NFSD_MAX_SLOTS_PER_SESSION)
+		     > PAGE_SIZE);
 
-	new = kzalloc(sizeof(*new) + mem, GFP_KERNEL);
+	new = kzalloc(struct_size(new, se_slots, numslots), GFP_KERNEL);
 	if (!new)
 		return NULL;
 	/* allocate each struct nfsd4_slot and data cache in one piece */