save/vfs: update comments wrt. tree/bupm ordering

save/vfs: update comments wrt. tree/bupm ordering

After looking into this and thinking about it, the comments here
are a bit misleading - save states the entries must be in a given
order without a rationale, and vfs states that the order is wrong
but gives an explanation that's not quite right.

Update both comments to make this clearer, and to document that
there's no inherent reason, just happened to pick something when
the save code was written, which turned out to be not the best.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>