debugfs_remove and debugfs_remove_recursive will check if the dentry
pointer is NULL or ERR, and will do nothing in that case.

Remove the check in cache_set_free and bch_debug_init.
Signed-off-by: NShenghui Wang <shhuiw@foxmail.com>
Signed-off-by: NColy Li <colyli@suse.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>