SUNRPC: temp var should match return type of xdr_skb_read_actor

The return type of xdr_skb_read_actor functions is size_t.  This fixes a
nit I unwittingly overlooked in commit dd456471.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>