NFS: Disambiguate nfs_stat_to_errno()

Rename the NFS4 version of nfs_stat_to_errno() so that it doesn't conflict with
the common one used by NFS2 and NFS3.

Signed-Off-By: David Howells <dhowells@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>