FROMLIST: arm64: mm: Fix and re-enable ARM64_SW_TTBR0_PAN

With the ASID now installed in TTBR1, we can re-enable ARM64_SW_TTBR0_PAN
by ensuring that we switch to a reserved ASID of zero when disabling
user access and restore the active user ASID on the uaccess enable path.

Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Tested-by: Shanker Donthineni <shankerd@codeaurora.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
(cherry picked from git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git


 commit 27a921e75711d924617269e0ba4adb8bae9fd0d1)
[ghackmann@google.com: adjust context, applying asm-uaccess.h changes to
 uaccess.h]
Signed-off-by: Greg Hackmann <ghackmann@google.com>
Change-Id: Iec7bff74f0605cea58793f2d1cb0d6546fa571e2
Git-Commit: 4ea2126b
Git-repo: git://android.googlesource.com/kernel/common.git


[vinmenon@codeaurora.org: trivial merge conflicts]
Signed-off-by: Vinayak Menon <vinmenon@codeaurora.org>