Only regular inodes are allowed for file-backed mounts, not directories
(as seen in the original syzbot case) or special inodes.

Also ensure that .read_folio() is implemented on the underlying fs
for the primary device.

Fixes: fb176750 ("erofs: add file-backed mount support")
Reported-by: syzbot+001306cd9c92ce0df23f@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/r/00000000000011bdde0622498ee3@google.com
Tested-by: syzbot+001306cd9c92ce0df23f@syzkaller.appspotmail.com
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240917130803.32418-1-hsiangkao@linux.alibaba.com