riscv: Fix memory_limit for 64-bit kernel

As described in Documentation/riscv/vm-layout.rst, the end of the
virtual address space for 64-bit kernel is occupied by the modules/BPF/
kernel mappings so this actually reduces the amount of memory we are able
to map and then use in the linear mapping. So make sure this limit is
correctly set.

Signed-off-by: Alexandre Ghiti <alex@ghiti.fr>
Fixes: 2bfc6cd8

 ("riscv: Move kernel mapping outside of linear mapping")
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>