tools/virtio: check mmap return value in vringh_test

tools/virtio: check mmap return value in vringh_test

In parallel_test(), the return values of mmap() for both host_map and
guest_map are not checked against MAP_FAILED. If mmap() fails, the
subsequent code will dereference the invalid pointer, leading to a
segmentation fault.

Add MAP_FAILED checks after both mmap() calls, using err() to report
the error and exit, consistent with the existing error handling style
in this file (e.g., the open() call on line 149).

Fixes: 1515c5ce26ae ("tools/virtio: add vring_test.")
Signed-off-by: longlong yan <yanlonglong@kylinos.cn>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20260605021446.1611-1-yanlonglong@kylinos.cn>