pktgen: Allow configuration of IPv6 source address range

Pktgen can use only one IPv6 source address from output device or src6
command setting. In pressure test we need create lots of sessions more
than 65535. So add src6_min and src6_max command to set the range.

Signed-off-by: Niu Xilei <niu_xilei@163.com>

Changes since v3:
 - function set_src_in6_addr use static instead of static inline
 - precompute min_in6_l,min_in6_h,max_in6_h,max_in6_l in setup time
Changes since v2:
 - reword subject line
Changes since v1:
 - only create IPv6 source address over least significant 64 bit range

Signed-off-by: David S. Miller <davem@davemloft.net>