riscv: Make vmalloc/vmemmap end equal to the start of the next region

We used to define VMALLOC_END equal to the start of the next region
*minus one* which is inconsistent with the use of this define in the
core code (for example, see the definitions of VMALLOC_TOTAL and
is_vmalloc_addr).

And then make the definition of VMEMMAP_END consistent with VMALLOC_END
and all other regions actually.

Signed-off-by: Alexandre Ghiti <alexandre.ghiti@canonical.com>
Reviewed-by: Jisheng Zhang <jszhang@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>