ARM: Use kernel pmap as intermediate mapping in context switch.
On ARM, we can directly switch between translation tables only when
the size of the mapping for any given virtual address is the same in
the old and new translation tables. The load of new TTB and subsequent
TLB flush is not atomic operation. So speculative page table walk can
load TLB entry from new mapping while rest of TLB entries are still the
old ones. In worst case, this can lead to situation when TLB cache can
contain multiple matching TLB entries. One (from old mapping) L2 entry
for VA + 4k and one (from new mapping) L1 entry for VA.

Thus, we must switch to kernel pmap translation table as intermediate
mapping because all sizes of these (old pmap and kernel pmap) mappings
are same (or unmapped). The same is true for switch from kernel pmap
translation table to new pmap one.