isisd: fix ipv6 metric endianness
the isis ipv6 reachability metric is transmitted in big endian / network
format, but isis_spf_process_lsp() does not convert this into host endian
format when mucking around with local cost + received metric. This patch
fixes this problem and makes received ipv6 metrics work properly on
little-endian machines.
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>