xfs_db: disable modification for CRC enabled filessytems.

xfs_db: disable modification for CRC enabled filessytems.

xfs_db does not have the IO infrastructure to calculate metadata
CRCs after modifying metadata. Hence xfs_db can only run in
read-only mode on filesystems with version 5 superblocks.

To fix this, xfs_db needs to have it's IO engine converted to use
the buffer based IO provided by libxfs rather than rolling it's own
IO routines. That is future work, so until this conversion is done,
only allow xfs_db to run in read-only mode on v5 filesystems.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>