sunrpc: on successful gss error pipe write, don't return error

When handling the gssd downcall, the kernel should distinguish between a
successful downcall that contains an error code and a failed downcall
(i.e. where the parsing failed or some other sort of problem occurred).

In the former case, gss_pipe_downcall should be returning the number of
bytes written to the pipe instead of an error. In the event of other
errors, we generally want the initiating task to retry the upcall so
we set msg.errno to -EAGAIN. An unexpected error code here is a bug
however, so BUG() in that case.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Cc: stable@kernel.org
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>