UPSTREAM: erofs: don't use erofs_map_blocks() any more

Currently, erofs_map_blocks() will be called only from
erofs_{bmap, read_raw_page} which are all for uncompressed files.
So, the compression branch in erofs_map_blocks() is pointless. Let's
remove it and use erofs_map_blocks_flatmode() directly. Also update
related comments.

Link: https://lore.kernel.org/r/20210325071008.573-1-zbestahu@gmail.com


Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Yue Hu <huyue2@yulong.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>

Bug: 201372112
Change-Id: Ic3362c62d7c917f05f1cd11120ee6280a22221b8
(cherry picked from commit 8137824e)
Signed-off-by: Huang Jianan <huangjianan@oppo.com>