ARM: mm: avoid taking ASID spinlock on fastpath

When scheduling a new mm, we take a spinlock so that we can:

  1. Safely allocate a new ASID, if required
  2. Update our active_asids field without worrying about parallel
     updates to reserved_asids
  3. Ensure that we flush our local TLB, if required

However, this has the nasty affect of serialising context-switch across
all CPUs in the system. The usual (fast) case is where the next mm has
a valid ASID for the current generation. In such a scenario, we can
avoid taking the lock and instead use atomic64_xchg to update the
active_asids variable for the current CPU. If a rollover occurs on
another CPU (which would take the lock), when copying the active_asids
into the reserved_asids another atomic64_xchg is used to replace each
active_asids with 0. The fast path can then detect this case and fall
back to spinning on the lock.

Tested-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>