blkid: recognize ext3 with test_fs set as ext3

blkid: recognize ext3 with test_fs set as ext3

It seems that if we have the test_filesystem flag set on an ext3
filesystem(!) on a system which provides ext4, blkid gets confused.

According to the current logic:

* It's not an ext4dev filesystem, because the system provides ext4.
* It's not an ext4 filesystem, because it has no ext4 features.
* It's not an ext3 filesystem, because the test flag is set.

In the end, it's nothing.

blkid should return *something* that is mountable... I'm inclined to
think that ext3 should be the right answer, if no ext4-specific features
are set.

This would mean just dropping the EXT2_FLAGS_TEST_FILESYS test in
probe_ext3(), because ext4 & ext4dev probes have come first already.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>