BACKPORT: ARM: uaccess: provide uaccess_save_and_enable() and uaccess_restore()

Provide uaccess_save_and_enable() and uaccess_restore() to permit
control of userspace visibility to the kernel, and hook these into
the appropriate places in the kernel where we need to access
userspace.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

Change-Id: I89c68368df268cbefa169e9d647b12c27b281c0f
(cherry picked from commit 3fba7e23f754a9a6e639b640fa2a393712ffe1b8)
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>