xfrm: reduce struct sec_path size

xfrm: reduce struct sec_path size

The mentioned struct has an hole and uses unnecessary wide type to
store MAC length and indexes of very small arrays.

It's also embedded into the skb_extensions, and the latter, due
to recent CAN changes, may exceeds the 192 bytes mark (3 cachelines
on x86_64 arch) on some reasonable configurations.

Reordering and the sec_path fields, shrinking xfrm_offload.orig_mac_len
to 16 bits and xfrm_offload.{len,olen,verified_cnt} to u8, we can save
16 bytes and keep skb_extensions size under control.

Before:

struct sec_path {
	int                        len;
	int                        olen;
	int                        verified_cnt;

	/* XXX 4 bytes hole, try to pack */$
	struct xfrm_state *        xvec[6];
	struct xfrm_offload ovec[1];

	/* size: 88, cachelines: 2, members: 5 */
	/* sum members: 84, holes: 1, sum holes: 4 */
	/* last cacheline: 24 bytes */
};

After:

struct sec_path {
	struct xfrm_state *        xvec[6];
	struct xfrm_offload        ovec[1];
	/* typedef u8 -> __u8 */ unsigned char              len;
	/* typedef u8 -> __u8 */ unsigned char              olen;
	/* typedef u8 -> __u8 */ unsigned char              verified_cnt;

	/* size: 72, cachelines: 2, members: 5 */
	/* padding: 1 */
	/* last cacheline: 8 bytes */
};

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Steffen Klassert <steffen.klassert@secunet.com>
Link: https://patch.msgid.link/83846bd2e3fa08899bd0162e41bfadfec95e82ef.1770398071.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>