xfs_copy: band-aids for CRC filesystems

xfs_copy: band-aids for CRC filesystems

xfs_copy needs a fair bit of work for CRCs because it rewrites
UUIDs by default, but this change will get it working properly
with the "-d" (duplicate) option which keeps the same UUID.

Accept the the CRC magic valid, change the ASSERT() to an error
message and exit more gracefully, and don't
even get started if we don't have the '-d' option.

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