linux: ensure that group dir fd is closed

The CLOEXEC flag on the fd will ensure that the directory is closed on
the child, but also need to close the fd in the parent.

src/os/cgroup.zig

@@ -102,6 +102,7 @@ pub fn cloneInto(cgroup: []const u8) !posix.pid_t {
         }
     };
     assert(fd >= 0);
+    defer _ = linux.close(fd);
 
     const args: extern struct {
         flags: u64,