erofs: fix file-backed mounts over FUSE

In the Linux kernel, the following vulnerability has been resolved:

erofs: fix file-backed mounts over FUSE

syzbot reported a null-ptr-deref in fuse_read_args_fill:

fuse_read_folio+0xb0/0x100 fs/fuse/file.c:905

filemap_read_folio+0xc6/0x2a0 mm/filemap.c:2367

do_read_cache_folio+0x263/0x5c0 mm/filemap.c:3825

read_mapping_folio include/linux/pagemap.h:1011 [inline]

erofs_bread+0x34d/0x7e0 fs/erofs/data.c:41

erofs_read_superblock fs/erofs/super.c:281 [inline]

erofs_fc_fill_super+0x2b9/0x2500 fs/erofs/super.c:625

Unlike most filesystems, some network filesystems and FUSE need

unavoidable valid `file` pointers for their read I/Os [1].

Anyway, those use cases need to be supported too.

[1] https://docs.kernel.org/filesystems/vfs.html