fuse: decrement NR_WRITEBACK_TEMP on the right page

NR_WRITEBACK_TEMP is accounted on the temporary page in the request, not
the page cache page.

Fixes: 8b284dc4

 ("fuse: writepages: handle same page rewrites")
Cc: <stable@vger.kernel.org> # v3.13
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>