netfilter: nf_conntrack: allow nf_ct_alloc_hashtable() to get highmem pages

commit ea781f19 (use SLAB_DESTROY_BY_RCU and get rid of call_rcu())
did a mistake in __vmalloc() call in nf_ct_alloc_hashtable().

I forgot to add __GFP_HIGHMEM, so pages were taken from LOWMEM only.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Patrick McHardy <kaber@trash.net>