FS/NFS: replace count*size kzalloc by kcalloc

kcalloc manages count*sizeof overflow.

Cc: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: linux-nfs@vger.kernel.org
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>