bcachefs: Change copygc wait amount to be min of per device waits

bcachefs: Change copygc wait amount to be min of per device waits

We're seeing a filesystem get stuck when all devices but one have no
more reclaimable buckets - because the copygc wait amount is curretly
filesystem wide.

This patch should fix that, possibly at the expensive of running too
much when only one or a few devices is full and the rebalance thread
needs to move data around.

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