extensions: libxt_tcpmss: Detect invalid ranges

extensions: libxt_tcpmss: Detect invalid ranges

Previously, an MSS range of e.g. 65535:1000 was silently accepted but
would then never match a packet since the kernel checks whether the MSS
value is greater than or equal to the first *and* less than or equal to
the second value.

Detect this as a parameter problem and update the man page accordingly.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>