xfrm: check for xdo_dev_state_free

The current XFRM code assumes that we've implemented the
xdo_dev_state_free() callback, even if it is meaningless to the driver.
This patch adds a check for it before calling, as done in other APIs,
to prevent a NULL function pointer kernel crash.

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