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