xfrm: restore GSO for SW crypto

Commit 49431af6 incorrectly assumes that the GSO path is only used
by HW offload, but it's also useful for SW crypto.

This patch re-enables GSO for SW crypto. It's not an exact revert to
preserve the other changes made to xfrm_dev_offload_ok afterwards, but
it reverts all of its effects.

Fixes: 49431af6 ("xfrm: rely on XFRM offload")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Reviewed-by: Zhu Yanjun <yanjun.zhu@linux.dev>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>