Make mountd complain on unsupported uid mapping schemes

Make mountd complain on unsupported uid mapping schemes

  The NFS kernel server does not support uid mappings, activated with flags
  such as "map_daemon" in exports. There is already code that parses these
  flags, and gives an error at mount time if an unsupported flag (ie. any
  but the default) is given. However, at some point the kernel changed the
  export interface, and the new code forgot to include the relevant check.
  Thus, simply copy the check from the old to the new code, which makes
  sure mountd behaves the same in this aspect regardless of kernel version,
  and makes sure the admin does not inadvertently use map_daemon and expect
  it to work.

Signed-off-by: Steinar H. Gunderson <sesse@debian.org>