bcachefs: Fix for copygc getting stuck waiting for reserve to be filled

bcachefs: Fix for copygc getting stuck waiting for reserve to be filled

This fixes a regression from the patch
  bcachefs: Fix copygc dying on startup

In general only the allocator thread itself should be updating
ca->allocator_state, the thread waking up the allocator setting it is an
ugly hack only needed to avoid racing with the copygc threads when we're
first starting up.

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