MFC	r262346:
ext2fs: fully enable ext4 read-only support.

The ext4 developers tend to tag Ext4-specific flags as
"incompatible" even when such features are not relevant for
read-only support.  This is a consequence of the process
though which this filesystem is implemented without design
and the fact that some new features are not extensible to
ext2/3.

Organize the features according to what we support and sort
them so that we can now read-only mount filesystems with
some features that may be found in newly formatted ext4 fs.

Submitted by:	Zheng Liu