bcachefs: Start copygc when first going read-write

bcachefs: Start copygc when first going read-write

In the distant past, it wasn't possible to start copygc until after
journal replay had finished. Now, the btree iterator code overlays keys
from the journal, so there's no reason not to start it earlier - and it
solves a rare deadlock.

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