rtnetlink: fix missing size for IFLA_IF_NETNSID

The size for IFLA_IF_NETNSID is missing from the size calculation
because the proceeding semicolon was not removed. Fix this by removing
the semicolon.

Detected by CoverityScan, CID#1461135 ("Structurally dead code")

Fixes: 79e1ad14

 ("rtnetlink: use netnsid to query interface")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>