x86, vdso: Unmap vdso pages

We mapped vdso pages but never unmapped them and the virtual address
is lost after exiting from the function, so unmap vdso pages here.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
LKML-Reference: <20100802004934.GA2505@sli10-desk.sh.intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>