Description: bpf: fix incorrect sign extension in check_alu_op()
References:
 https://bugs.chromium.org/p/project-zero/issues/detail?id=1454
 http://www.openwall.com/lists/oss-security/2017/12/21/2
 https://github.com/brl/grlh/blob/master/get-rekt-linux-hardened.c
Notes:
 carnil> Intorduced in 4.9-rc1 with 484611357c19f9e19ef742ebef4505a07d243cc9
Bugs:
upstream: released (4.15-rc5) [95a762e2c8c942780948091f8f2a4f32fce1ac6f]
4.9-upstream-stable: released (4.9.72) [3695b3b18519099224efbc5875569d2cb6da256d]
3.16-upstream-stable: N/A "Vulnerable code introduced in 4.9-rc1"
3.2-upstream-stable: N/A "Vulnerable code introduced in 4.9-rc1"
sid: released (4.14.7-1) [bugfix/all/bpf-fix-incorrect-sign-extension-in-check_alu_op.patch]
4.9-stretch-security: released (4.9.65-3+deb9u1) [bugfix/all/bpf-fix-incorrect-sign-extension-in-check_alu_op.patch]
3.16-jessie-security: N/A "Vulnerable code introduced later"
3.2-wheezy-security: N/A "Vulnerable code introduced later"