syscall: update check for UserNS support for centos 7
syscall: update check for UserNS support for centos 7

Fixes #20796
Fixes #16283

Change-Id: Ib11992fbd2bc1fbb3b14ae5a6bf0da2e4c12f641
Reviewed-on: https://go-review.googlesource.com/49311
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
Run-TryBot: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

cloudflare/go,polyverse-security/go,golang/go,josharian/go,cloudflare/go,Cofyc/go,cloudflare/go,CAFxX/go,nawawi/go,josharian/go,codestation/go,polyverse-security/go,vsekhar/elastic-go,cloudflare/go,vsekhar/elastic-go,polyverse-security/go,Cofyc/go,josharian/go,CAFxX/go,polyverse-security/go,vsekhar/elastic-go,codestation/go,christopher-henderson/Go,polyverse-security/go,golang/go,golang/go,josharian/go,josharian/go,codestation/go,akutz/go,golang/go,golang/go,cloudflare/go,golang/go,codestation/go,nawawi/go,nawawi/go,CAFxX/go,polyverse-security/go,CAFxX/go,Cofyc/go,polyverse-security/go,golang/go,akutz/go,CAFxX/go,vsekhar/elastic-go,akutz/go,codestation/go,golang/go,Cofyc/go,josharian/go,cloudflare/go,CAFxX/go,codestation/go,nawawi/go,akutz/go,cloudflare/go,Cofyc/go,akutz/go,josharian/go,vsekhar/elastic-go,nawawi/go,akutz/go,josharian/go,CAFxX/go,nawawi/go,vsekhar/elastic-go,Cofyc/go,Cofyc/go,Cofyc/go,CAFxX/go,vsekhar/elastic-go,cloudflare/go,nawawi/go,akutz/go,codestation/go,polyverse-security/go,vsekhar/elastic-go,codestation/go,akutz/go,nawawi/go