Add Loop Device package

This adds Loop device support in our Linux kernel and adds a Go package for working with them.
It also drive-by adds a pre-mounted tmpfs to ktest as that is quite useful in a lot of situations.

Test Plan: Comes with ktests.

X-Origin-Diff: phab/D745
GitOrigin-RevId: fa06bcdddc033efb136f56da3b4a91159273bf88
diff --git a/metropolis/test/ktest/init/main.go b/metropolis/test/ktest/init/main.go
index f6049db..0236531 100644
--- a/metropolis/test/ktest/init/main.go
+++ b/metropolis/test/ktest/init/main.go
@@ -39,6 +39,7 @@
 		{"/proc", "proc", unix.MS_NOEXEC | unix.MS_NOSUID | unix.MS_NODEV},
 		{"/dev", "devtmpfs", unix.MS_NOEXEC | unix.MS_NOSUID},
 		{"/dev/pts", "devpts", unix.MS_NOEXEC | unix.MS_NOSUID},
+		{"/tmp", "tmpfs", 0},
 	} {
 		if err := os.Mkdir(el.dir, 0755); err != nil && !os.IsExist(err) {
 			return fmt.Errorf("could not make %s: %w", el.dir, err)