xfrm: fix policy unreferencing on larval drop

I mistakenly had the error path to use num_pols to decide how
many policies we need to drop (cruft from earlier patch set
version which did not handle socket policies right).

This is wrong since normally we do not keep explicit references
(instead we hold reference to the cache entry which holds references
to policies). drop_pols is set to num_pols if we are holding the
references, so use that. Otherwise we eventually BUG_ON inside
xfrm_policy_destroy due to premature policy deletion.

Signed-off-by: Timo Teras <timo.teras@iki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>