riscv: Fix set_direct_map_default_noflush() to reset _PAGE_EXEC

When resetting the linear mapping permissions, we must make sure that we
clear the X bit so that do not end up with WX mappings (since we set
PAGE_KERNEL).

Fixes: 395a21ff ("riscv: add ARCH_HAS_SET_DIRECT_MAP support")
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20231213134027.155327-3-alexghiti@rivosinc.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>