netlink: Eliminate kmalloc in netlink dump operation.

Following patch stores struct netlink_callback in netlink_sock
to avoid allocating and freeing it on every netlink dump msg.
Only one dump operation is allowed for a given socket at a time
therefore we can safely convert cb pointer to cb struct inside
netlink_sock.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-repo: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git


Git-commit: 16b304f3

Change-Id: I13a3cdf8b0e8e0adb1e386eca105a86cb540d2fa
Signed-off-by: Srinivasa Rao Kuppala <srkupp@codeaurora.org>