netfs: Pass a pointer to virt_to_page()

Like the other calls in this function virt_to_page() expects
a pointer, not an integer.

However since many architectures implement virt_to_pfn() as
a macro, this function becomes polymorphic and accepts both a
(unsigned long) and a (void *).

Fix this up with an explicit cast.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>