net: hsr: serialize seq_blocks merge across nodes

net: hsr: serialize seq_blocks merge across nodes

During node merging, hsr_handle_sup_frame() walks node_curr->seq_blocks
to update node_real without holding node_curr->seq_out_lock. This
allows concurrent mutations from duplicate registration paths, risking
inconsistent state or XArray/bitmap corruption.

Fix this by locking both nodes' seq_out_lock during the merge.
To prevent ABBA deadlocks, locks are acquired in order of memory
address.

Reviewed-by: Felix Maurer <fmaurer@redhat.com>
Fixes: 415e6367512b ("hsr: Implement more robust duplicate discard for PRP")
Signed-off-by: Luka Gejak <luka.gejak@linux.dev>
Link: https://patch.msgid.link/20260401092243.52121-2-luka.gejak@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>