NFS: Use the "nfs_stat" enum for nfs_stat_to_errno()'s argument
Clean up.
To distinguish more clearly between the on-the-wire NFSERR_ value and
our local errno values, use the proper type for the argument of
nfs_stat_to_errno().
Add a documenting comment appropriate for a global function shared
outside this source file.
Signed-off-by: Chuck Lever <chuck.lever@oracle.com> Tested-by: J. Bruce Fields <bfields@redhat.com> Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>