bcachefs: Fix a couple null ptr derefs when no disk groups exist

Normally successfully parsing a target means disk groups should exist,
but we don't want a BUG() or null ptr deref if we end up with an invalid
target.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/disk_groups.c

@@ -183,7 +183,7 @@ const struct bch_devs_mask *bch2_target_to_mask(struct bch_fs *c, unsigned targe
 	case TARGET_GROUP: {
 		struct bch_disk_groups_cpu *g = rcu_dereference(c->disk_groups);
 
-		return t.group < g->nr && !g->entries[t.group].deleted
+		return g && t.group < g->nr && !g->entries[t.group].deleted
 			? &g->entries[t.group].devs
 			: NULL;
 	}
@@ -208,7 +208,7 @@ bool bch2_dev_in_target(struct bch_fs *c, unsigned dev, unsigned target)
 
 		rcu_read_lock();
 		g = rcu_dereference(c->disk_groups);
-		m = t.group < g->nr && !g->entries[t.group].deleted
+		m = g && t.group < g->nr && !g->entries[t.group].deleted
 			? &g->entries[t.group].devs
 			: NULL;
 

fs/bcachefs/io.c

@@ -55,7 +55,9 @@ static bool bch2_target_congested(struct bch_fs *c, u16 target)
 		return false;
 
 	rcu_read_lock();
-	devs = bch2_target_to_mask(c, target);
+	devs = bch2_target_to_mask(c, target) ?:
+		&c->rw_devs[BCH_DATA_user];
+
 	for_each_set_bit(d, devs->d, BCH_SB_MEMBERS_MAX) {
 		ca = rcu_dereference(c->devs[d]);
 		if (!ca)