[dev] Increase the number of MPLS tags in zebra

By the way, kernel 4.12 is already available: (https://cdn.kernel.org/pub/linux/kernel/v4.x/linux-4.12.tar.xz) In this kernel, according to the developers, the number of MPLS tags on the stack is 30. Therefore, I suggest that this limit in Zebra should be avoided. https://github.com/FRRouting/frr/issues/776