Just like with TCP and UDP protocol matches, emit a simple 'meta
l4proto' match if no specific header detail is to be matched.
Note that plain '-m sctp' should be a NOP in kernel, but '-p sctp -m
sctp' is not and the translation is deferred to the extension in that
case. Keep things stu^Wsimple and translate unconditionally.
Reviewed-by: Florian Westphal <fw@strlen.de> Signed-off-by: Phil Sutter <phil@nwl.cc>