net: reorder flowi_common fields to avoid holes

the flowi* structures are used and memsetted by server functions
in critical path. Currently flowi_common has a couple of holes that
we can eliminate reordering the struct fields. As a side effect,
both flowi4 and flowi6 shrink by 8 bytes.

Before:
pahole -EC flowi_common
struct flowi_common {
// ...
	/* size: 40, cachelines: 1, members: 10 */
	/* sum members: 32, holes: 1, sum holes: 4 */
	/* padding: 4 */
	/* last cacheline: 40 bytes */
};
pahole -EC flowi6
struct flowi6 {
// ...
        /* size: 88, cachelines: 2, members: 6 */
        /* padding: 4 */
        /* last cacheline: 24 bytes */
};
pahole -EC flowi4
struct flowi4 {
// ...
        /* size: 56, cachelines: 1, members: 4 */
        /* padding: 4 */
        /* last cacheline: 56 bytes */
};

After:
struct flowi_common {
// ...
	/* size: 32, cachelines: 1, members: 10 */
	/* last cacheline: 32 bytes */
};
struct flowi6 {
// ...
        /* size: 80, cachelines: 2, members: 6 */
        /* padding: 4 */
        /* last cacheline: 16 bytes */
};
struct flowi4 {
// ...
        /* size: 48, cachelines: 1, members: 4 */
        /* padding: 4 */
        /* last cacheline: 48 bytes */
};

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>