UPSTREAM: 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 commit 27a921e75711d924617269e0ba4adb8bae9fd0d1)

[toddpoynor@google.com: fixup context, move asm-uaccess.h changes to
uaccess.h]
Change-Id: Ie8e1706152b8d7c068c6b1f20241a10669a69ca1
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>