bcachefs: Shut down quicker

Internal writes (i.e. copygc/rebalance operations) shouldn't be blocking
on the allocator when we're going RO.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/io.c

@@ -1070,6 +1070,12 @@ static void __bch2_write(struct closure *cl)
 					BKEY_EXTENT_U64s_MAX))
 			goto flush_io;
 
+		if ((op->flags & BCH_WRITE_FROM_INTERNAL) &&
+		    percpu_ref_is_dying(&c->writes)) {
+			ret = -EROFS;
+			goto err;
+		}
+
 		wp = bch2_alloc_sectors_start(c,
 			op->target,
 			op->opts.erasure_code,