NFS: Clean up: copy hostname with kstrndup during mount processing

Clean up: mount option parsing uses kstrndup in several places, rather than
using kzalloc.  Replace the few remaining uses of kzalloc with kstrndup,
for consistency.

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