LTS: proc: fix lookup in /proc/net subdirectories after setns(2)

["Rantala, Tommi T. (Nokia - FI/Espoo)" <tommi.t.rantala@xxxxxxxxx>]

Hi Greg,

Can you cherry-pick these to 4.19.y & 5.4.y:

commit e06689bf57017ac022ccf0f2a5071f760821ce0f
Author: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Date:   Wed Dec 4 16:49:59 2019 -0800

    proc: change ->nlink under proc_subdir_lock

commit c6c75deda81344c3a95d1d1f606d5cee109e5d54
Author: Alexey Dobriyan <adobriyan@xxxxxxxxx>
Date:   Tue Dec 15 20:42:39 2020 -0800

    proc: fix lookup in /proc/net subdirectories after setns(2)


-Tommi