xfrm: state: fix sparse warnings in xfrm_state_init

Use rcu_assign_pointer, and tmp variables for freeing on the error
path without accessing net->xfrm.state_by*.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>