ARM: fix cache-xsc3l2 after stack based kmap_atomic()

Since commit 3e4d3af5 "mm: stack based kmap_atomic()", it is actively
wrong to rely on fixed kmap type indices (namely KM_L2_CACHE) as
kmap_atomic() totally ignores them and a concurrent instance of it may
happily reuse any slot for any purpose.  Because kmap_atomic() is now
able to deal with reentrancy, we can get rid of the ad hoc mapping here,
and we even don't have to disable IRQs anymore (highmem case).

While the code is made much simpler, there is a needless cache flush
introduced by the usage of __kunmap_atomic().  It is not clear if the
performance difference to remove that is worth the cost in code
maintenance (I don't think there are that many highmem users on that
platform if at all anyway).

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>