RISC-V: Fix usage of memblock_enforce_memory_limit

memblock_enforce_memory_limit accepts the maximum memory size not the
maximum address that can be handled by kernel. Fix the function invocation
accordingly.

Fixes: 1bd14a66

 ("RISC-V: Remove any memblock representing unusable memory area")
Cc: stable@vger.kernel.org
Reported-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>