bcachefs: chardev: make bch_chardev_class constant

bcachefs: chardev: make bch_chardev_class constant

Since commit 43a7206b0963 ("driver core: class: make class_register() take
a const *"), the driver core allows for struct class to be in read-only
memory, so move the bch_chardev_class structure to be declared at build
time placing it into read-only memory, instead of having to be dynamically
allocated at boot time. Also, correctly clean up after failing paths in
bch2_chardev_init().

Cc: Hongbo Li <lihongbo22@huawei.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Suggested-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ricardo B. Marliere <ricardo@marliere.net>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>