userns: close all file descriptors before entering into userns
userns: close all file descriptors before entering into userns

Cc: Jann Horn <1ff1d5ceb421971f93e6626b02a9591b29d7600e@thejh.net>
Reporte-by: Jann Horn <1ff1d5ceb421971f93e6626b02a9591b29d7600e@thejh.net>
Signed-off-by: Andrew Vagin <74c05afa918858348d6db8037d82fa8124870113@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

eabatalov/criu,tych0/criu,KKoukiou/criu-remote,AuthenticEshkinKot/criu,eabatalov/criu,tych0/criu,ldu4/criu,ldu4/criu,eabatalov/criu,efiop/criu,AuthenticEshkinKot/criu,AuthenticEshkinKot/criu,KKoukiou/criu-remote,ldu4/criu,ldu4/criu,eabatalov/criu,tych0/criu,eabatalov/criu,AuthenticEshkinKot/criu,tych0/criu,KKoukiou/criu-remote,efiop/criu,AuthenticEshkinKot/criu,efiop/criu,AuthenticEshkinKot/criu,KKoukiou/criu-remote,KKoukiou/criu-remote,efiop/criu,tych0/criu,KKoukiou/criu-remote,tych0/criu,ldu4/criu,efiop/criu,eabatalov/criu,ldu4/criu,efiop/criu