x86, pageattr: Prevent overflow in slow_virt_to_phys() for X86_PAE

pte_pfn() returns a PFN of long (32 bits in 32-PAE), so "long <<
PAGE_SHIFT" will overflow for PFNs above 4GB.

Due to this issue, some Linux 32-PAE distros, running as guests on Hyper-V,
with 5GB memory assigned, can't load the netvsc driver successfully and
hence the synthetic network device can't work (we can use the kernel parameter
mem=3000M to work around the issue).

Cast pte_pfn() to phys_addr_t before shifting.

Fixes: "commit d76565344512: x86, mm: Create slow_virt_to_phys()"
Signed-off-by: Dexuan Cui <decui@microsoft.com>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: gregkh@linuxfoundation.org
Cc: linux-mm@kvack.org
Cc: olaf@aepfle.de
Cc: apw@canonical.com
Cc: jasowang@redhat.com
Cc: dave.hansen@intel.com
Cc: riel@redhat.com
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/1414580017-27444-1-git-send-email-decui@microsoft.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

arch/x86/mm/pageattr.c

@@ -409,7 +409,7 @@ phys_addr_t slow_virt_to_phys(void *__virt_addr)
 	psize = page_level_size(level);
 	pmask = page_level_mask(level);
 	offset = virt_addr & ~pmask;
-	phys_addr = pte_pfn(*pte) << PAGE_SHIFT;
+	phys_addr = (phys_addr_t)pte_pfn(*pte) << PAGE_SHIFT;
 	return (phys_addr | offset);
 }
 EXPORT_SYMBOL_GPL(slow_virt_to_phys);