8021q: use RCU for egress QoS mappings

8021q: use RCU for egress QoS mappings

The TX fast path and reporting paths walk egress QoS mappings without
RTNL. Convert the mapping lists to RCU-protected pointers, use RCU
reader annotations in readers, and defer freeing mapping nodes with an
embedded rcu_head.

This prepares the egress QoS mapping code for safe removal of mapping
nodes in a follow-up change while preserving the current behavior.

Co-developed-by: Yuan Tan <yuantan098@gmail.com>
Signed-off-by: Yuan Tan <yuantan098@gmail.com>
Signed-off-by: Longxuan Yu <ylong030@ucr.edu>
Signed-off-by: Ren Wei <n05ec@lzu.edu.cn>
Link: https://patch.msgid.link/9136768189f8c6d3f824f476c62d2fa1111688e8.1776647968.git.yuantan098@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>