netfilter: nat: merge nf_nat_redirect into nf_nat

Similar to previous patch, this time, merge redirect+nat.
The redirect module is just 2k in size, get rid of it and make
redirect part available from the nat core.

before:
   text    data     bss     dec     hex filename
  19461    1484    4138   25083    61fb net/netfilter/nf_nat.ko
   1236     792       0    2028     7ec net/netfilter/nf_nat_redirect.ko
after:
  20340    1508    4138   25986    6582 net/netfilter/nf_nat.ko

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>