netfilter: remove unnecessary returns from void function()s

This patch removes from net/ netfilter files
all the unnecessary return; statements that precede the
last closing brace of void functions.

It does not remove the returns that are immediately
preceded by a label as gcc doesn't like that.

Done via:
$ grep -rP --include=*.[ch] -l "return;\n}" net/ | \
  xargs perl -i -e 'local $/ ; while (<>) { s/\n[ \t\n]+return;\n}/\n}/g; print; }'

Signed-off-by: Joe Perches <joe@perches.com>
[Patrick: changed to keep return statements in otherwise empty function bodies]
Signed-off-by: Patrick McHardy <kaber@trash.net>