[NETFILTER]: nf_conntrack: optimize __nf_conntrack_find()

Ignoring specific entries in __nf_conntrack_find() is only needed by NAT
for nf_conntrack_tuple_taken(). Remove it from __nf_conntrack_find()
and make nf_conntrack_tuple_taken() search the hash itself.

Saves 54 bytes of text in the hotpath on x86_64:

  __nf_conntrack_find      |  -54 # 321 -> 267, # inlines: 3 -> 2, size inlines: 181 -> 127
  nf_conntrack_tuple_taken | +305 # 15 -> 320, lexblocks: 0 -> 3, # inlines: 0 -> 3, size inlines: 0 -> 181
  nf_conntrack_find_get    |   -2 # 90 -> 88
 3 functions changed, 305 bytes added, 56 bytes removed, diff: +249

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>