Repeated overnight kernel builds in tmpfs showed insane Committed_AS
by morning.  The main bug was that shmem_file_write was passing
(newsize-oldsize)>>PAGE_SHIFT to vm_enough_memory, but it has to be
((newsize>>PAGE_SHIFT)-(oldsize>>PAGE_SHIFT)) - imagine 1k writes.

But actually, if we're going to do strict accounting, then we should
round up to next page not down - use VM_ACCT macro throughout (needs
unusual mix of PAGE_CACHE_SIZE with PAGE_SHIFT); and must count one
page for a long symlink.