nft: Speed up immediate parsing

nft: Speed up immediate parsing

Parsing of rules which jump to a chain pointlessly causes a call to
xtables_find_target() despite the code already knowing the outcome.

Avoid the significant delay for rulesets with many chain jumps by
performing the (standard) target lookup only for accept/drop/return
verdicts.

From a biased test-case on my VM:

| # iptables-nft-save | grep -c -- '-j'
| 133943
| # time ./old/iptables-nft-save >/dev/null
| real	0m45.566s
| user	0m1.308s
| sys	0m8.430s
| # time ./new/iptables-nft-save >/dev/null
| real	0m3.547s
| user	0m0.762s
| sys	0m2.476s

Signed-off-by: Phil Sutter <phil@nwl.cc>
Acked-by: Florian Westphal <fw@strlen.de>