UPSTREAM: arm64: mm: Allocate ASIDs in pairs

In preparation for separate kernel/user ASIDs, allocate them in pairs
for each mm_struct. The bottom bit distinguishes the two: if it is set,
then the ASID will map only userspace.

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 0c8ea531b7740754cf374ca8b7510655f569c5e3)

[toddpoynor@google.com: fixup context]
Change-Id: I9879f45cd51cdf13814e3cff18653a76c893f386
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>