netfilter: conntrack: use fallthrough to cleanup

These cases all use the same function. we can simplify the code through
fallthrough.

$ size net/netfilter/nf_conntrack_core.o

        text	   data	    bss	    dec	    hex	filename
before  81601	  81430	    768	 163799	  27fd7	net/netfilter/nf_conntrack_core.o
after   80361	  81430	    768	 162559	  27aff	net/netfilter/nf_conntrack_core.o

Arch: aarch64
Gcc : gcc version 9.4.0 (Ubuntu 9.4.0-1ubuntu1~20.04.1)

Reported-by: k2ci <kernel-bot@kylinos.cn>
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>