xen/grants: fix hypercall continuation for GNTTABOP_cache_flush

xen/grants: fix hypercall continuation for GNTTABOP_cache_flush

The GNTTABOP_cache_flush hypercall has a wrong test for hypercall
continuation, the test today is:

    if ( rc > 0 || opaque_out != 0 )

Unfortunately this will be true even in case of an error (rc < 0),
possibly leading to very long lasting hypercalls (times of more
than an hour have been observed in a test case).

Correct the test condition to result in false with rc < 0 and set
opaque_out only if no error occurred, to be on the safe side.

Partially-suggested-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>