netfilter: x_tables: rectify XT_FUNCTION_MAXNAMELEN usage

There has been quite a confusion in userspace about
XT_FUNCTION_MAXNAMELEN; because struct xt_entry_match used MAX-1,
userspace would have to do an awkward MAX-2 for maximum length
checking (due to '\0'). This patch adds a new define that matches the
definition of XT_TABLE_MAXNAMELEN - being the size of the actual
struct member, not one off.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Patrick McHardy <kaber@trash.net>