arm64: Fix map_range() not splitting mapped blocks

arm64: Fix map_range() not splitting mapped blocks

The implementation of map_range() creates the requested mapping by
walking the page tables, iterating over multiple PTEs and/or descending
into existing table mappings as needed. When doing so, it assumes any
pre-existing valid PTE to be a table mapping. This assumption is wrong
if the platform code attempts to successively map two overlapping ranges
where the latter intersects a block mapping created for the former.

As a result, map_range() treats the existing block mapping as a table
mapping and descends into it i.e. starts interpreting the
previously-mapped range as an array of PTEs, writing to them and
potentially even descending further (extra fun with MMIO ranges!).

Instead, pass any valid non-table mapping to split_block(), which
ensures that it actually was a block mapping (calls panic() otherwise)
before splitting it.

Fixes: 41e2787f5ec4 ("arm64: Reduce add_map() complexity")
Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>
Tested-by: Fabio Estevam <festevam@gmail.com>
Tested-by: Hiago De Franco <hiago.franco@toradex.com> # Toradex Verdin AM62
Reviewed-by: Marc Zyngier <maz@kernel.org>