xfs_repair: fix error in process_sf_dir2_fixi8

xfs_repair: fix error in process_sf_dir2_fixi8

The goal of process_sf_dir2_fixi8 is to convert an i8 shortform
directory into a (shorter) i4 shortform directory.  It achieves this by
duplicating the old sf directory contents (as oldsfp), zeroing i8count
in the caller's directory buffer (i.e. newsfp/sfp), and reinitializing
the new directory with the old directory's entries.

Unfortunately, it copies the parent pointer from sfp (the buffer we've
already started changing), not oldsfp.  This leads to directory
corruption since at that point we zeroed i8count, which means that we
save only the upper four bytes from the parent pointer entry.

This was found by fuzzing u3.sfdir3.hdr.i8count = ones in xfs/384.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>