netlink: add signed types

Signed types might be needed in NL communication from time to time
(I need s32 in team driver), so add them.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: 4778e0be16c291ba6d9d55eeff3a6764fc84a071
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
CRs-Fixed: 685019
Change-Id: I54db73ccd3da9967fd027cc0e95c7c2c4e66d8c2
Signed-off-by: Manjunathappa Prakash <prakashpm@codeaurora.org>