bcachefs: Improved copygc pipelining

bcachefs: Improved copygc pipelining

This improves copygc pipelining across multiple buckets: we now track
each in flight bucket we're evacuating, with separate moving_contexts.

This means that whereas previously we had to wait for outstanding moves
to complete to ensure we didn't try to evacuate the same bucket twice,
we can now just check buckets we want to evacuate against the pending
list.

This also mean we can run the verify_bucket_evacuated() check without
killing pipelining - meaning it can now always be enabled, not just on
debug builds.

This is going to be important for the upcoming erasure coding work,
where moving IOs that are being erasure coded will now skip the initial
replication step; instead the IOs will wait on the stripe to complete.

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