fix CVE-2014-4699 (ptrace vulnerability)

https://bugs.archlinux.org/task/41115
http://www.openwall.com/lists/oss-security/2014/07/04/4
https://github.com/torvalds/linux/commit/b9cd18de4db3c9ffa7e17b0dc0ca99ed5aa4d43a
bump 3.15.4
upgpkg: linux-linode 3.15.4-1