macsec: use rcu_work to defer RX SA crypto cleanup out of softirq

macsec: use rcu_work to defer RX SA crypto cleanup out of softirq

crypto_free_aead() can internally invoke vunmap() (e.g. via
dma_free_attrs() in hardware crypto drivers such as hisi_sec2).
vunmap() must not be called from softirq context, but free_rxsa()
is an RCU callback that runs in softirq, leading to a kernel crash:

  vunmap+0x4c/0x70
  __iommu_dma_free+0xd0/0x138
  dma_free_attrs+0xf4/0x100
  sec_aead_exit+0x64/0xb8 [hisi_sec2]
  crypto_destroy_tfm+0x98/0x110
  free_rxsa+0x28/0x50 [macsec]
  rcu_do_batch+0x184/0x460
  rcu_core+0xf4/0x1f8
  handle_softirqs+0x118/0x330

Use rcu_work to defer the cleanup to a workqueue. rcu_work dispatches
the worker asynchronously after the RCU grace period, so no thread
blocks waiting, and concurrent releases of multiple SAs naturally
share the same grace period.

Fixes: c09440f7dcb3 ("macsec: introduce IEEE 802.1AE driver")
Signed-off-by: Jinliang Zheng <alexjlzheng@tencent.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20260511153102.2640368-3-alexjlzheng@tencent.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>