progress: relabel the inaccurate ETA column

progress: relabel the inaccurate ETA column

ETA is calculated using a sample obtained by snooping on bees's normal
crawling operations.

This sample is heavily biased and not representative of the entire
filesystem.  If the distribution of extent sizes in the filesystem is
not uniform, the ETA can be wildly wrong.

Collecting an accurate sample set would require extra IO and CPU time
which should be spent doing dedupes instead.

Explicitly label the ETA as inaccurate to avoid having too many users
report the same bug.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>