staging: erofs: add super block operations

This commit adds erofs super block operations, including (u)mount,
remount_fs, show_options, statfs, in addition to some private
icache management functions.

Signed-off-by: Miao Xie <miaoxie@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>