runtime: check that sysUnused is always physical-page aligned

If sysUnused is passed an address or length that is not aligned to the
physical page boundary, the kernel will unmap more memory than the
caller wanted. Add a check for this.

For #9993.

Change-Id: I68ff03032e7b65cf0a853fe706ce21dc7f2aaaf8
Reviewed-on: https://go-review.googlesource.com/22065Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Dave Cheney <dave@cheney.net>
Reviewed-by: Michael Hudson-Doyle <michael.hudson@canonical.com>

src/runtime/mem_linux.go

@@ -132,6 +132,13 @@ func sysUnused(v unsafe.Pointer, n uintptr) {
 		}
 	}
 
+	if uintptr(v)&(sys.PhysPageSize-1) != 0 || n&(sys.PhysPageSize-1) != 0 {
+		// madvise will round this to any physical page
+		// *covered* by this range, so an unaligned madvise
+		// will release more memory than intended.
+		throw("unaligned sysUnused")
+	}
+
 	madvise(v, n, _MADV_DONTNEED)
 }