xfs_db: stop misusing an onstack inode

xfs_db: stop misusing an onstack inode

The onstack inode in xfs_check's process_inode is a potential landmine
since it's not a /real/ incore inode.  The upcoming 5.8 merge will make
this messier wrt inode forks, so just remove the onstack inode and
reference the ondisk fields directly.  This also reduces the amount of
thinking that I have to do w.r.t. future libxfs porting efforts.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>