If we support mmap MAP_NORESERVE, we should support it on shared
anonymous objects: too bad that needs a few changes.  do_mmap_pgoff pass
VM_ACCOUNT (or not) down to shmem_file_setup, flag stored into shmem
info, for use by shmem_delete_inode later.  Also removed a harmless but
pointless call to shmem_truncate.