A privileged user in a super block's s_user_ns is privileged
towards that file system and thus should be allowed to set file
capabilities. The file capabilities will not be trusted outside
of s_user_ns, so an unprivileged user cannot use this to gain
privileges in a user namespace where they are not already
privileged.
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>