mm: handle THP in swap_*page_fs()

Pages passed to swap_readpage()/swap_writepage() are not necessarily all
the same size - there may be transparent-huge-pages involves.

The BIO paths of swap_*page() handle this correctly, but the SWP_FS_OPS
path does not.

So we need to use thp_size() to find the size, not just assume PAGE_SIZE,
and we need to track the total length of the request, not just assume it
is "page * PAGE_SIZE".

Link: https://lkml.kernel.org/r/165119301488.15698.9457662928942765453.stgit@noble.brown
Signed-off-by: NeilBrown <neilb@suse.de>
Reported-by: Miaohe Lin <linmiaohe@huawei.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: David Howells <dhowells@redhat.com>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Hugh Dickins <hughd@google.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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