zebra: use larger buffer (4096) for netlink messages

zebra: use larger buffer (4096) for netlink messages

Increase the maximum number of nexthops per route by using a larger
buffer for netlink messages.

  * zebra/rt_netlink.c: Use a buffer of 4096 bytes to parse/build
    netlink messages. Add a hash define for this number
    (NL_PKT_BUF_SIZE). Most places in the code were previously using a
    buffer of 1024 bytes.