db: add a special attribute buffer verifier

db: add a special attribute buffer verifier

Because we only have a single attribute type that is used for all
the attribute buffer types, we need to provide a special verifier
for the read code. That verifier needs to know all the attribute
types and when it find one it knows about, switch to the correct
verifier and call it.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rich Johnston <rjohnston@sgi.com>