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: fb176750266a ("erofs: add file-backed mount support")
Reported-by: syzbot+001306cd9c92ce0df23f@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/r/00000000000011bdde0622498ee3@xxxxxxxxxx
Tested-by: syzbot+001306cd9c92ce0df23f@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>