bcachefs: Add missing bch2_moving_ctxt_flush_all()

commit ef740a1e

 upstream.

This fixes a bug with rebalance IOs getting stuck with reads completed,
but writes never being issued.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>