staging: erofs: kill sbi->dev_name

As Al said, "the only use of sbi->dev_name is debugging
printks and all of those have sb->s_id available, with
device name stored in there.  Which makes the whole
thing bloody weird".

sbi->dev_name was used for our debugging use and it's
better to just use s_id in community and delete
the whole erofs_mount_private stuff.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Link: https://lore.kernel.org/r/20190731155752.210602-11-gaoxiang25@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>