repair: fix the variable-width nlink array

repair: fix the variable-width nlink array

It looks like we currently never grow the variable-width nlink array
if only the on-disk nlink size overflows 8 bits.  This leads to a major
mess in nlink counting, and eventually an assert in phase7.

Replace the indirect all mess with a union that allows doing proper
array arithmetics while we're at it.

Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>