bcachefs: Get ref on c->writes in move.c

bcachefs: Get ref on c->writes in move.c

There's no point reading an extent in order to move it if the write is
going to fail because we're shutting down. This patch changes the move
path so that moving_io now owns a ref on c->writes - as a bonus,
rebalance and copygc will now notice that we're shutting down and exit
quicker.

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