netns: ip netns exec <name> kill doesn't make sense

It seems confusing netns with pidns.
Although 'ip netns exec' doesn't make sense,
'ip netns exec <netns> kill <pid>' itself success as expected.
But as side effects, dentry of /proc/<pid>/ns/net becomes young,
which increases the possibility to fail to delete netns. That's not good.

Fixes: Bug #1158589
Change-Id: I9aa717ccb86d8bf00bb1e707d39bfb65d043532b
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Merge "netns: ip netns exec <name> kill doesn't make sense"