pppoe: optimize hash with word access

pppoe: optimize hash with word access

Currently, hash_item() processes the 6-byte Ethernet address and the
2-byte session ID byte-wise to compute a hash.

Optimize this by using 16-bit word operations: XOR three 16-bit words
from the Ethernet address and the 16-bit session ID, then fold the
result. This reduces the total number of loads and XORs. The Ethernet
addresses in a skb and struct pppoe_addr are both 2-byte aligned, so the
u16 pointer cast is safe.

Signed-off-by: Qingfang Deng <qingfang.deng@linux.dev>
Link: https://patch.msgid.link/20260429023848.153425-1-qingfang.deng@linux.dev
Signed-off-by: Jakub Kicinski <kuba@kernel.org>