Patch from Hugh Dickins <hugh@veritas.com>

tmpfs failed fsx+swapout tests after many hours, a page found zeroed.
Not a truncate problem, but mirror image of earlier truncate problems:
swap goes through mpage_writepages, which must therefore allow for a
sudden swizzle back to file identity.

Second time this caught us, so I've audited the tree for other places
which might be surprised by such swizzling.  The only others I found
were (perhaps) in the parisc and sparc64 flush_dcache_page called
from do_generic_mapping_read on a looped tmpfs file which is also
mmapped; but that's a very marginal case, I wanted to understand it
better before making any edit, and now realize that hch's sendfile
in loop eliminates it (now go through do_shmem_file_read instead:
similar but crucially this locks the page when raising its count,
which is enough to keep vmscan from interfering).