xfrm: cleanup error path in xfrm_add_policy()

Replace the open-coded manual cleanup in the error path of
xfrm_add_policy() with xfrm_policy_destroy(), which already
handles all the necessary cleanup internally. This is consistent
with how xfrm_policy_construct() handles its own error paths.

The walk.dead flag must be set before calling xfrm_policy_destroy()
as required by BUG_ON(!policy->walk.dead).

Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>