x86/vdso: Set VM_MAYREAD for the vvar vma

The VVAR area can, obviously, be read; that is kind of the point.

AFAIK this has no effect whatsoever unless x86 suddenly turns into a
nommu architecture.  Nonetheless, not setting it is suspicious.

Reported-by: Nathan Lynch <Nathan_Lynch@mentor.com>
Signed-off-by: Andy Lutomirski <luto@amacapital.net>
Link: http://lkml.kernel.org/r/e4c8bf4bc2725bda22c4a4b7d0c82adcd8f8d9b8.1406330779.git.luto@amacapital.net
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>