bcachefs: Fix copygc dying on startup

bcachefs: Fix copygc dying on startup

The copygc threads errors out and makes the filesystem go RO if it ever
tries to run and discovers it has no reserve allocated - which is a
problem if it races with the allocator thread and its reserve hasn't
been filled yet.

The allocator thread doesn't start filling the copygc reserve until
after BCH_FS_STARTED has been set, so make sure to wake up the allocator
threads after setting that and before starting copygc.

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