Patch from Hugh Dickins

Added shmem_readpage and shmem_prepare_write so tmpfs files can be used
by the loop driver (together with simple_commit_write).  shmem_getpage
extended to accept file page passed in, which may have to be copied
over from swap page.

Use bdget and sb_set_blocksize so loop can see our preferred blocksize
PAGE_CACHE_SIZE.  Use copy_highpage, removed from highmem.h in 2.4.17:
restore it but with kmap_atomics.  Restore (a simple) copy_page to
asm-sparc64/page.h, which alone of all arches discarded it.