[pax] Updated to patch 3.15.8-test6

Updated to pax-linux-3.15.8-test6.patch

PaX Team says:
  - fixed an incompatiblity between STACKLEAK and Xen, reported by joe (http://forums.grsecurity.net/viewtopic.php?f=3&t=3997#p14241)
  - fixed the ESPFIX SS limit on i386 so that it actually works instead of triggering a double fault, reported by Oscon