bcachefs: Redo copygc throttling

bcachefs: Redo copygc throttling

The code that checked the current free space and waited if it was too
big was causing issues - btree node allocations do not increment the
write IO clock (perhaps they should); but more broadly the check
wouldn't run copygc at all until the device was mostly full, at which
point it might have to do a bunch of work.

This redoes that logic so that copygc starts to run earlier, smoothly
running more and more often as the device becomes closer to full.

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