Patch from Hugh Dickins

Checked shmem_file_write against recent filemap source, and against
2.4 and 2.4-ac: folded in missing fixes, mostly related to far file
positions.  Plus the new kmap_atomic copying technique.  But for now,
as before, no mark_page_accessed or SetPageReferenced in shmem.c: add
those, or whatever, later on when akpm has reviewed usage elsewhere.