Revert "Revert "arm: mm: Add VM_ARM_EMPTY_MAPPING flag to mark the PMD gaps""

This reverts commit 22ab06efcc1e3941b713da081b53a8d03a26e13c.
Revert "ARM: 7527/1: uaccess: explicitly check __user pointer when !CPU_USE_DOMAINS"

This reverts commit 8cc876def310b034ab0e0775a14d1a49472d7f5f.