core: fix the keepalive condition in close session

According to the The GP Internal Core API v1.1:
The keepalive flag should be ignored when the single instance flag is
not set.

Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>
Signed-off-by: Zeng Tao <prime.zeng@hisilicon.com>

core/kernel/tee_ta_manager.c

@@ -373,6 +373,7 @@ TEE_Result tee_ta_close_session(struct tee_ta_session *csess,
 {
  struct tee_ta_session *sess;
  struct tee_ta_ctx *ctx;
+ bool keep_alive;
 
  DMSG("tee_ta_close_session(0x%" PRIxVA ")", (vaddr_t)csess);
 
@@ -416,7 +417,9 @@ TEE_Result tee_ta_close_session(struct tee_ta_session *csess,
  panic();
 
  ctx->ref_count--;
- if (!ctx->ref_count && !(ctx->flags & TA_FLAG_INSTANCE_KEEP_ALIVE)) {
+ keep_alive = (ctx->flags & TA_FLAG_INSTANCE_KEEP_ALIVE) &&
+ (ctx->flags & TA_FLAG_SINGLE_INSTANCE);
+ if (!ctx->ref_count && !keep_alive) {
  DMSG(" ... Destroy TA ctx");
 
  TAILQ_REMOVE(&tee_ctxes, ctx, link);