Make correct mask when converting IPv4 <=> IPv6

[Tietew]

When converting IPv4 to/from IPv6, IPAddr#set generates incorrect mask value.
This causes IPAddr#include? returns incorrect result.

ip4 = IPAddr.new('::ffff:127.0.0.1').native
p [ip4.to_s, ip4.prefix]
ip6 = IPAddr.new('127.0.0.1').ipv4_mapped
p [ip6.to_s, ip6.prefix]

Expected

["127.0.0.1", 32]
["::ffff:127.0.0.1", 128]

Actual

["127.0.0.1", -96]       # @mask_addr == 0xFFFFFFFFFFFFFFFF
["::ffff:127.0.0.1", 0]  # @mask_addr == 0x000000000000FFFF

[ioquatix]

@Tietew can you rebase on master.

[Tietew]

@ioquatix done.

[ioquatix]

Can you please squash typo fix.