riscv: Don't use PGD entries for the linear mapping

[ Upstream commit 629db01c

 ]

Propagating changes at this level is cumbersome as we need to go through
all the page tables when that happens (either when changing the
permissions or when splitting the mapping).

Note that this prevents the use of 4MB mapping for sv32 and 1GB mapping for
sv39 in the linear mapping.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20231108075930.7157-2-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
Stable-dep-of: c67ddf59

 ("riscv: force PAGE_SIZE linear mapping if debug_pagealloc is enabled")
Signed-off-by: Sasha Levin <sashal@kernel.org>