powerpc/powernv: Fix vma page prot flags in opal-prd driver

opal-prd driver will mmap() firmware code/data area as private
mapping to prd user space daemon.  Write to this page will
trigger COW faults.  The new COW pages are normal kernel RAM
pages accounted by the kernel and are not special.

vma->vm_page_prot value will be used at page fault time
for the new COW pages, while pgprot_t value passed in
remap_pfn_range() is used for the initial page table entry.

Hence:
* Do not add _PAGE_SPECIAL in vma, but only for remap_pfn_range()
* Also remap_pfn_range() will add the _PAGE_SPECIAL flag using
  pte_mkspecial() call, hence no need to specify in the driver

This fix resolves the page accounting warning shown below:
BUG: Bad rss-counter state mm:c0000007d34ac600 idx:1 val:19

The above warning is triggered since _PAGE_SPECIAL was incorrectly
being set for the normal kernel COW pages.

Signed-off-by: Vaidyanathan Srinivasan <svaidy@linux.vnet.ibm.com>
Acked-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>