bcachefs: fix error path of __bch2_read_super()

In __bch2_read_super(), if kstrdup() fails, it needs to release memory
in sb->holder, fix to call bch2_free_super() in the error path.

Signed-off-by: Chao Yu <chao@kernel.org>
Reviewed-by: Hongbo Li <lihongbo22@huawei.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>