ext4: support freezing ext2 (nojournal) file systems

Through an oversight, when we added nojournal support to ext4, we
didn't add support to allow file system freezing.  This is relatively
easy to add, so let's do it.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reported-by: Dexuan Cui <decui@microsoft.com>