Patch from Hugh Dickins

Here I intended a patch to remove the unsatisfactory shmem_recalc_inode
(which tries to work out when vmscan has freed undirtied hole pages,
to relax its blocks-in-use limit; but can only do so per-inode when it
needs to be per-super).  I had hoped to use the releasepage method, but
it looks like it can't quite be bent to this task, the page might still
be rebusied after release.  2.4-ac uses a removepage method dedicated to
this, but I'm reluctant to ask for such a minor address_space_operation.

So, leave shmem_recalc_inode as is, but avoid the issue as much as
possible, by letting shmem_getpage use the empty_zero_page instead of
allocating when a hole is read (but this cannot be done when it's being
mapped, nowadays the nopage doesn't know if page will be copied or not).
Whereupon shmem_getpage(,,,SGP_READ) can do partial trunc's holdpage.