iscsi-target: Convert iscsi_session statistics to atomic_long_t

This patch converts a handful of iscsi_session statistics to type
atomic_long_t, instead of using iscsi_session->session_stats_lock
when incrementing these values.

More importantly, go ahead and drop the spinlock usage within
iscsit_setup_scsi_cmd(), iscsit_check_dataout_hdr(),
iscsit_send_datain(), and iscsit_build_rsp_pdu() fast-path code.

(Squash in Roland's  target: Remove write-only stats fields and lock
from struct se_node_acl)

Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>

drivers/target/iscsi/iscsi_target_util.c

@@ -980,7 +980,7 @@ static void iscsit_handle_nopin_response_timeout(unsigned long data)
 		tiqn->sess_err_stats.last_sess_failure_type =
 				ISCSI_SESS_ERR_CXN_TIMEOUT;
 		tiqn->sess_err_stats.cxn_timeout_errors++;
-		conn->sess->conn_timeout_errors++;
+		atomic_long_inc(&conn->sess->conn_timeout_errors);
 		spin_unlock_bh(&tiqn->sess_err_stats.lock);
 	}
 	}