bcachefs: Fix refcounting in discard path

bch_dev->io_ref does not protect against the filesystem going away;
bch_fs->writes does.

Thus the filesystem write ref needs to be the last ref we release.

Reported-by:  <syzbot+9e0404b505e604f67e41@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>