xfrm: Return detailed errors from xfrmi_newlink

Currently all failure modes of xfrm interface creation return EEXIST.
This change improves the granularity of errnos provided by also
returning ENODEV or EINVAL if failures happen in looking up the
underlying interface, or a required parameter is not provided.

This change has been tested against the Android Kernel Networking Tests,
with additional xfrmi_newlink tests here:

https://android-review.googlesource.com/c/kernel/tests/+/715755



Signed-off-by: Benedict Wong <benedictwong@google.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>