xfrm: Fix crash when the hold queue is used.

The commits "xfrm: Move dst->path into struct xfrm_dst"
and "net: Create and use new helper xfrm_dst_child()."
changed xfrm bundle handling under the assumption
that xdst->path and dst->child are not a NULL pointer
only if dst->xfrm is not a NULL pointer. That is true
with one exception. If the xfrm hold queue is used
to wait until a SA is installed by the key manager,
we create a dummy bundle without a valid dst->xfrm
pointer. The current xfrm bundle handling crashes
in that case. Fix this by extending the NULL check
of dst->xfrm with a test of the DST_XFRM_QUEUE flag.

Fixes: 0f6c480f ("xfrm: Move dst->path into struct xfrm_dst")
Fixes: b92cf4aa ("net: Create and use new helper xfrm_dst_child().")
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>