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: NChuck Lever <chuck.lever@oracle.com>
Tested-by: NJ. Bruce Fields <bfields@redhat.com>
Signed-off-by: NTrond Myklebust <Trond.Myklebust@netapp.com>