bcachefs: Change bch2_dev_lookup() to not use lookup_bdev()

bch2_dev_lookup() is used from the extended attribute set methods, for
setting the target options, where we're already holding an inode lock -
it turns out pathname lookups also take inode locks, so that was
susceptible to deadlocks.

Fortunately we already stash the device name in ca->name. This does
change user-visible behaviour though: instead of specifying e.g.
/dev/sda1, user must now specify sda1.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

fs/bcachefs/super.c

@@ -1883,20 +1883,14 @@ int bch2_dev_resize(struct bch_fs *c, struct bch_dev *ca, u64 nbuckets)
 }
 
 /* return with ref on ca->ref: */
-struct bch_dev *bch2_dev_lookup(struct bch_fs *c, const char *path)
+struct bch_dev *bch2_dev_lookup(struct bch_fs *c, const char *name)
 {
 	struct bch_dev *ca;
-	dev_t dev;
 	unsigned i;
-	int ret;
-
-	ret = lookup_bdev(path, &dev);
-	if (ret)
-		return ERR_PTR(ret);
 
 	rcu_read_lock();
 	for_each_member_device_rcu(ca, c, i, NULL)
-		if (ca->dev == dev)
+		if (!strcmp(name, ca->name))
 			goto found;
 	ca = ERR_PTR(-ENOENT);
 found: