Later we're going to try the self-contained image verification.
The current superblock checksum feature has quite limited
functionality, instead, merkle trees can provide better protection
for image integrity.
xxhash is also used in the following xattr name filter feature. It is
redundant for one filesystem to rely on two hashing algorithms at the
same time.
Since the superblock checksum is a compatible feature, just deprecate
it now.
Signed-off-by: Jingbo Xu <jefflexu@xxxxxxxxxxxxxxxxx>