iscsit: use GFP_ATOMIC under spin lock

The function iscsit_build_conn_drop_async_message() is called
from iscsit_close_connection() with spin lock 'sess->conn_lock'
held, so we should use GFP_ATOMIC instead of GFP_KERNEL.
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Cc: stable@vger.kernel.org
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target.c

@@ -2336,7 +2336,7 @@ static void iscsit_build_conn_drop_async_message(struct iscsi_conn *conn)
 	if (!conn_p)
 		return;
 
-	cmd = iscsit_allocate_cmd(conn_p, GFP_KERNEL);
+	cmd = iscsit_allocate_cmd(conn_p, GFP_ATOMIC);
 	if (!cmd) {
 		iscsit_dec_conn_usage_count(conn_p);
 		return;