[SRU][Bionic/Artful] fix false positives in W+X checking

[Manoj Iyer]

Please consider this patch to Bionic, Artful and apply to Cosmic. On
ARM64 system from Cavium and Qualcomm we see random false positive
warning messages wrt W+X checking.  "arm64/mm: Found insecure W+X mapping at address 0000000000a99000/0xa99000" while booting.

A kernel with the upstream patch is avaliable in ppa:manjo/lp1769696,
the patch was cleanly cherry-picked from linux-next on to bionic and
also cleanly applies to Artful. I tested the kernel on a QTI QDF2400 and
Cavium ThunderX system. Since we cannot reliably and consistently
reproduce the warning, I did not see the warning after doing repeated
reboots with stock bionic.