libpod: don't make a broken symlink for /etc/mtab on FreeBSD

This file has not been present in BSD systems since 2.9.1 BSD and as far
as I remember /proc/mounts has never existed on BSD systems.

[NO NEW TESTS NEEDED]

Signed-off-by: Doug Rabson <dfr@rabson.org>

libpod/container_internal_linux.go

@@ -784,3 +784,20 @@ func (c *Container) safeMountSubPath(mountPoint, subpath string) (s *safeMountIn
 		mountPoint: npath,
 	}, nil
 }
+
+func (c *Container) makePlatformMtabLink(etcInTheContainerFd, rootUID, rootGID int) error {
+	// If /etc/mtab does not exist in container image, then we need to
+	// create it, so that mount command within the container will work.
+	err := unix.Symlinkat("/proc/mounts", etcInTheContainerFd, "mtab")
+	if err != nil && !os.IsExist(err) {
+		return fmt.Errorf("creating /etc/mtab symlink: %w", err)
+	}
+	// If the symlink was created, then also chown it to root in the container
+	if err == nil && (rootUID != 0 || rootGID != 0) {
+		err = unix.Fchownat(etcInTheContainerFd, "mtab", rootUID, rootGID, unix.AT_SYMLINK_NOFOLLOW)
+		if err != nil {
+			return fmt.Errorf("chown /etc/mtab: %w", err)
+		}
+	}
+	return nil
+}