NFS: Use kzalloc() instead of kmalloc() in the idmapper

This will allocate memory that has already been zeroed, allowing us to
remove the memset later on.

Signed-off-by: Bryan Schumaker <bjchuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>