namespaces: use fstatat instead of readlink to get a namespace kernel id
namespaces: use fstatat instead of readlink to get a namespace kernel id

It should be faster and we don't need to parse a string.

Signed-off-by: Andrew Vagin <74c05afa918858348d6db8037d82fa8124870113@virtuozzo.com>
Acked-by: Cyrill Gorcunov <7a1ea01eee6961eb1e372e3508c2670446d086f4@openvz.org>
Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@virtuozzo.com>

eabatalov/criu,KKoukiou/criu-remote,eabatalov/criu,KKoukiou/criu-remote,KKoukiou/criu-remote,eabatalov/criu,eabatalov/criu,KKoukiou/criu-remote,KKoukiou/criu-remote,eabatalov/criu,eabatalov/criu,KKoukiou/criu-remote