fuse: decrement NR_WRITEBACK_TEMP on the right page

fuse: decrement NR_WRITEBACK_TEMP on the right page

BugLink: https://bugs.launchpad.net/bugs/1824553
NR_WRITEBACK_TEMP is accounted on the temporary page in the request, not
the page cache page.

Fixes: 8b284dc47291 ("fuse: writepages: handle same page rewrites")
Cc: <stable@vger.kernel.org> # v3.13
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
(cherry picked from commit a2ebba824106dabe79937a9f29a875f837e1b6d4)
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>