bcachefs: Also log overwrites in journal

Lately we've been doing a lot of debugging by looking at the journal to
see what was changed, and by what code path. This patch adds a new
journal entry type for recording overwrites, so that we don't have to
search backwards through the journal to see what was being overwritten
in order to work out what the triggers were supposed to be doing.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>