netns: fix NLM_F_ECHO mechanism for RTM_NEWNSID

[Nicolas Dichtel]

Hi Ubuntu kernel team,

please, can you consider to cherry-pick the following linux upstream fix:
993e4c929a07 ("netns: fix NLM_F_ECHO mechanism for RTM_NEWNSID").

It has been included in v5.4 and the patch that introduces the bug has been
pushed in v4.0.

Here is the launchpad entry:
https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1849085


Thank you,
Nicolas