bcachefs: Fix locking in bch2_ioc_setlabel()

Fixes: 7a254053 ("bcachefs: support FS_IOC_SETFSLABEL")
Reported-by: syzbot+7e9efdfec27fbde0141d@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/fs-ioctl.c

@@ -328,9 +328,8 @@ static int bch2_ioc_setlabel(struct bch_fs *c,
 
 	mutex_lock(&c->sb_lock);
 	strscpy(c->disk_sb.sb->label, label, BCH_SB_LABEL_SIZE);
-	mutex_unlock(&c->sb_lock);
-
 	ret = bch2_write_super(c);
+	mutex_unlock(&c->sb_lock);
 
 	mnt_drop_write_file(file);
 	return ret;