RISC-V: Fix building error when CONFIG_SPARSEMEM_MANUAL=y

Fix a build break by adjusting where VMALLOC_* and FIXADDR_* are
defined.  This fixes the definition of the MEMMAP_* macros.

  CC      init/main.o
In file included from ./include/linux/mm.h:99,
                 from ./include/linux/ring_buffer.h:5,
                 from ./include/linux/trace_events.h:6,
                 from ./include/trace/syscall.h:7,
                 from ./include/linux/syscalls.h:85,
                 from init/main.c:21:
./arch/riscv/include/asm/pgtable.h: In function ‘pmd_page’:
./arch/riscv/include/asm/pgtable.h:95:24: error: ‘VMALLOC_START’ undeclared (first use in this function); did you mean ‘VMEMMAP_START’?
 #define VMEMMAP_START (VMALLOC_START - VMEMMAP_SIZE)
                        ^~~~~~~~~~~~~

Fixes: d95f1a54 ("RISC-V: Implement sparsemem")
Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
[paul.walmsley@sifive.com: minor patch description fix]
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>