Attached is a patch which implements pthread based multi-threading in 
xfs_repair.

For a small file system, on Irix, I observed the following run times:

With no optimization: 10 minutes 21 seconds
With multi-threading (only): 7 minutes 35 seconds
With lio_listio pre-fetching (only): 3 minutes 59 seconds
With both multi-threading and lio_listio pre-fetching: 3 minutes and 14 
seconds.


For a large fs:
With no optimizations: ~36 hours
With both multi-threading and pre-fetching: ~7 hours.

Thanks.

/Madan Valluri