staging: erofs: keep up erofs_fs.h with erofs-outofstaging patchset

The main change is to reserve all checksums except for superblock,
since it's more useful to do block-based verity for read-only fs.

Some comments change as well, which is minor.

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190731155752.210602-5-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>