xfrm: replace deprecated strncpy with strscpy_pad

The function strncpy is deprecated since it does not guarantee the
destination buffer is NULL terminated. Recommended replacement is
strscpy. The padded version was used to remain consistent with the other
strscpy_pad usage in the modified function.

Signed-off-by: Daniel Yang <danielyangkang@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>