xfs_db: don't hardcode 'type data' size at 512b

xfs_db: don't hardcode 'type data' size at 512b

On a disk with 4096-byte LBAs, the xfs_db 'type data' subcommand doesn't
work:

# xfs_io -c 'sb' -c 'type data' /dev/sda
xfs_db: read failed: Invalid argument
no current object

The cause of this is the hardcoded initialization of bb_count when we're
setting type data -- it should be the filesystem sector size, not just 1.

Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>