xfrm: fix freed block size calculation in xfrm_policy_fini()

Missing multiplication of block size by sizeof(struct hlist_head)
can cause xfrm_hash_free() to be called with wrong second argument
so that kfree() is called on a block allocated with vzalloc() or
__get_free_pages() or free_pages() is called with wrong order when
a namespace with enough policies is removed.

Bug introduced by commit a35f6c5d

, i.e. versions >= 2.6.29 are
affected.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>