drivers/chown: use pwalk.Walk
drivers/chown: use pwalk.Walk

This is a parallel Walk implementation and we're expecting some
speedup thanks to that.

Note that pwalk.Walk error handling is primitive; for one thing,
it never sends an error to the callpack, thus the error checking
code is no longer needed.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

rhatdan/storage,containers/storage,rhatdan/storage,containers/storage,containers/storage,rhatdan/storage