zebra: fix netlink NL_PKT_BUF_SIZE

zebra: fix netlink NL_PKT_BUF_SIZE

Change default value of variable NL_PKT_BUF_SIZE to 8192UL.  Cf.
NLMSG_GOODSIZE definition of linux in include/linux/netlink.h for detail.

Previously, on platforms with a page size greater than 8192, if you had added
too many interfaces, zebra would not have enough buffer space to get the entire
interface list.  This resulted in an incomplete interface list.

From: 高鹏 <gpstrive@gmail.com>
[updated to apply after FPM patches]
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>