xfs_db: convert directory parsing to use libxfs structure

xfs_db: convert directory parsing to use libxfs structure

xfs_db rolls it's own "opaque" directory types for the different
block formats. All it cares about is where the headers end and the
data starts, and none of the other details in the structures. Rather
than duplicate this for the dir3 format, we already have perfectly
good headers and abstraction functions for finding this information
in libxfs.  Using these means that the dir2 code used for printing
fields, metadump and check need to be modified to use libxfs
definitions.

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