riscv: Make sure the linear mapping does not use the kernel mapping

For 64-bit kernel, the end of the address space is occupied by the
kernel mapping and currently, the functions to populate the kernel page
tables (i.e. create_p*d_mapping) do not override existing mapping so we
must make sure the linear mapping does not map memory in the kernel mapping
by clipping the memory above the memory limit.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Fixes: c9811e37

 ("riscv: Add mem kernel parameter support")
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>