svcrpc: fix failures to handle -1 uid's

As of f025adf1 "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.

Commit afe3c3fd "svcrpc: fix failures to
handle -1 uid's and gid's" fixed part of the problem, but overlooked the
gid upcall--the kernel can request supplementary gid's for the -1 uid,
but mountd's attempt write a response will get -EINVAL.

Symptoms were nfsd failing to reply to the first attempt to use a newly
negotiated krb5 context.

Reported-by: Sven Geggus <lists@fuchsschwanzdomain.de>
Tested-by: Sven Geggus <lists@fuchsschwanzdomain.de>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>