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ext4 metadata_csum and backwards compatibility

There are some Enterprise and Data-centre scenarios where this may cause
some pain.

1. iSCSI SAN where target and initiator are using incompatible versions
of e2fsprogs
2. LVM SAN (sanlock or DLM) where host and clients are using
incompatible versions

In both cases the problem arises where SAN host and client have
incompatible versions of e2fsprogs and at some point both need to do
file system management.

For example, a stable 16.04 LTS SAN host may be set to do regular fsck
operations on block devices exported over the network to prevent
saturating the network, as well as utilising the host's native I/O

If the exported block devices have been formatted by a 16.04 client then
the operations on the 16.04 SAN host may fail.

In reverse, there's an issue where a 18.04 SAN host formats the block
devices but 16.04 clients do routine boot/mount-time fsck.

I'd favour an SRU to 16.04.