vhost-vdpa: set v->config_ctx to NULL if eventfd_ctx_fdget() fails

In vhost_vdpa_set_config_call() if eventfd_ctx_fdget() fails the
'v->config_ctx' contains an error instead of a valid pointer.

Since we consider 'v->config_ctx' valid if it is not NULL, we should
set it to NULL in this case to avoid to use an invalid pointer in
other functions such as vhost_vdpa_config_put().

Fixes: 776f3950

 ("vhost_vdpa: Support config interrupt in vdpa")
Cc: lingshan.zhu@intel.com
Cc: stable@vger.kernel.org
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Link: https://lore.kernel.org/r/20210311135257.109460-3-sgarzare@redhat.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>