netns: enable to dump full nsid translation table

Like the previous patch, the goal is to ease to convert nsids from one
netns to another netns.
A new attribute (NETNSA_CURRENT_NSID) is added to the kernel answer when
NETNSA_TARGET_NSID is provided, thus the user can easily convert nsids.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/uapi/linux/net_namespace.h

@@ -17,6 +17,7 @@ enum {
 	NETNSA_PID,
 	NETNSA_FD,
 	NETNSA_TARGET_NSID,
+	NETNSA_CURRENT_NSID,
 	__NETNSA_MAX,
 };