netlink-socket: Fix sign of error code.

Commit 8f20fd98db (netlink-socket: Work around upstream kernel Netlink
bug.) got the sign of the error code wrong, so that it reported e.g. -22
for EINVAL to nl_sock_recv__()'s caller, instead of 22.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Alex Wang <alexw@nicira.com>