[IPV6]: Make xfrm6_init to return an error code.

The xfrm initialization function does not return any error code, so if
there is an error, the caller can not be advise of that.  This patch
checks the return code of the different called functions in order to
return a successful or failed initialization.

Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>
Acked-by: Benjamin Thery <benjamin.thery@bull.net>
Signed-off-by: David S. Miller <davem@davemloft.net>