powerpc: map vma key-protection bits to pte key bits.

Map  the  key  protection  bits of the vma to the pkey bits in
the PTE.

The PTE  bits used  for pkey  are  3,4,5,6  and 57. The  first
four bits are the same four bits that were freed up  initially
in this patch series. remember? :-) Without those four bits
this patch wouldn't be possible.

BUT, on 4k kernel, bit 3, and 4 could not be freed up. remember?
Hence we have to be satisfied with 5, 6 and 7.

Signed-off-by: Ram Pai <linuxram@us.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>