neighbour: fix unaligned access to pneigh_entry

After the blamed commit, the member key is longer 4-byte aligned. On
platforms that do not support unaligned access, e.g., MIPS32R2 with
unaligned_action set to 1, this will trigger a crash when accessing
an IPv6 pneigh_entry, as the key is cast to an in6_addr pointer.

Change the type of the key to u32 to make it aligned.

Fixes: 62dd9318 ("[IPV6] NDISC: Set per-entry is_router flag in Proxy NA.")
Signed-off-by: Qingfang DENG <qingfang.deng@siflower.com.cn>
Link: https://lore.kernel.org/r/20230601015432.159066-1-dqfext@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/neighbour.h

@@ -180,7 +180,7 @@ struct pneigh_entry {
 	netdevice_tracker	dev_tracker;
 	u32			flags;
 	u8			protocol;
-	u8			key[];
+	u32			key[];
 };
 
 /*