Use uid/gid of -1 to indicate the export's anonuid/anongid should be used

Kernel routine nfsd_setuser() in fs/nfsd/auth.c checks for the
value -1 and defaults the credential's fsuid/fsgid to the
correct anonuid/anongid values for the given export.  We should
be passing this value (-1) down when a name mapping cannot be found.
Thanks to J. Bruce Fields <bfields@fieldses.org> for the reference.

Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
Acked-by: J. Bruce Fields <bfields@fieldses.org>