inet: shrink struct flowi_common

inet: shrink struct flowi_common

I am looking at syzbot reports triggering kernel stack overflows
involving a cascade of ipvlan devices.

We can save 8 bytes in struct flowi_common.

This patch alone will not fix the issue, but is a start.

Fixes: 24ba14406c5c ("route: Add multipath_hash in flowi_common to make user-define hash")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: wenxu <wenxu@ucloud.cn>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20231025141037.3448203-1-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>