Specify the filesystem type when mounting to /host (#10169)

When mounting the partition that contains `/host` during initramfs, the
mount binary available there (coming from busybox) tries each filesystem
in `/proc/filesystems` and sees which one succeeds. During this time,
there may be some error messages logged into dmesg because some of the
incorrect filesystems failed to mount the partition.

Specify the filesystem type explicitly so that initramfs knows it's that
type, and we know what filesystem will always get used there.

Fixes #9998

Signed-off-by: Saikrishna Arcot <sarcot@microsoft.com>

files/initramfs-tools/union-mount.j2

@@ -132,7 +132,7 @@ case "${ROOT}" in
         ;;
     *)
         ## Mount the raw partition again
-        mount ${ROOT} ${rootmnt}/host
+        mount -t ext4 ${ROOT} ${rootmnt}/host
         ;;
 esac