Patch from Hugh Dickins

Remove obsolete shmem_fs_type: we were in some doubt whether safe yet
to do so, then found 2.5.4 typo changed it from 2.4's "shm" to "shmem"
ever since: nobody complained, delete it - we're "tmpfs" since 2.4.4.
Use libfs' simple_empty and simple_sync_file instead of homegrown.
Remove exit_shmem_fs, it fools people that this might be a module.
Allow for faint possibility that shm_mnt could not be initialized.