iscsi->fd is never initialized in iser driver, so iscsi_disconnect
always does not work for iser context.
iscsi->fd is used as a member variable of TCP context, so let iscsi
TCP driver handle iscsi->fd, we just call iscsi_disconnect in
iscsi_destroy_context. Luckly, TCP driver has already handle invalid
iscsi->fd case in iscsi_tcp_disconnect.
And fix NULL pointer case for iscsi_disconnect.
Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>