xt_geoip: fix in6_addr little-endian byte swapping

xt_geoip: fix in6_addr little-endian byte swapping

The Perl script that builds the GeoIP DBs uses inet_pton(3) to convert
the addresses to network byte order. This converts

  "1234:5678::90ab:cdef"

to:

  0x12 0x34 0x56 0x78 .. 0xcd 0xef, interpreted by an LE machine
  accessing this in uint32_t-sized chunks as
  8765:4321::fedc:ba09

The kernel module compares the addresses in packets with the ranges from
the DB in host byte order using binary search. It uses 32-bit swaps
when converting the addresses.

libxt_geoip, however, which the module uses to load the ranges from the
DB and convert them from NBO to HBO, uses 16-bit swaps to do so, and
this means that:

  1234:5678::90ab:cdef

becomes:

  4321:8765::ba09:fedc

Obviously, this is inconsistent with the kernel module and DB build
script and breaks the binary search.

Fixes: b91dbd03c717 ("geoip: store database in network byte order")
Reported-by: "Thomas B. Clark" <kernel@clark.bz>
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>