db: verify buffer on type change

db: verify buffer on type change

Currently when the type command is run, we simply change the type
associated with the buffer, but don't verify it. This results in
unchecked CRCs being displayed. Hence when changing the type, run
the verifier associated with the type to determine if the buffer
contents is valid or not.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>