svcrpc: fix failures to handle -1 uid's and gid's

svcrpc: fix failures to handle -1 uid's and gid's

As of f025adf191924e3a75ce80e130afcd2485b53bb8 "sunrpc: Properly decode
kuids and kgids in RPC_AUTH_UNIX credentials" any rpc containing a -1
(0xffff) uid or gid would fail with a badcred error.

Reported symptoms were xmbc clients failing on upgrade of the NFS
server; examination of the network trace showed them sending -1 as the
gid.

Reported-by: Julian Sikorski <[email protected]>
Tested-by: Julian Sikorski <[email protected]>
Cc: "Eric W. Biederman" <[email protected]>
Cc: [email protected]
Signed-off-by: J. Bruce Fields <[email protected]>