We're expanding hugetlbfs i_size in the wrong place.  If someone attempts to
mmap more pages than are available, i_size is updated to reflect the
attempted mapping size.

So set i_size only when pages are successfully added to the mapping.

i_size handling at truncate time is still a bit wrong - if the mapping has
pages at (say) page offset 100-200 and the mappng is truncated to (say) page
offset 50, i_size should be set to zero.  But it is instead set to
50*HPAGE_SIZE.  That's harmless.