torture: Make torture.sh print the number of files to be compressed

Compressing gigabyte vmlinux files can take some time, and it can be a
bit annoying to not know many more batches of compression there will be.
This commit therefore makes torture.sh print the number of files to be
compressed just before starting compression and just after compression
completes.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>