xfrm: check for xdo_dev_ops add and delete

This adds a check for the required add and delete functions up front
at registration time to be sure both are defined.

Since both the features check and the registration check are looking
at the same things, break out the check for both to call.

Lastly, for some reason the feature check was setting xfrmdev_ops to
NULL if the NETIF_F_HW_ESP bit was missing, which would probably
surprise the driver later if the driver turned its NETIF_F_HW_ESP bit
back on.  We shouldn't be messing with the driver's callback list, so
we stop doing that with this patch.

Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>