riscv: mm: Define DIRECT_MAP_PHYSMEM_END

riscv: mm: Define DIRECT_MAP_PHYSMEM_END

On RISC-V, the actual mappable range of physical address space is
dependent on the current MMU mode i.e. satp_mode (See
Documentation/arch/riscv/vm-layout.rst).

Define the DIRECT_MAP_PHYSMEM_END macro based on the existing virtual
address space layout macros to expose this information to
get_free_mem_region(). Otherwise, it returns a region that couldn't be
mapped, which breaks ZONE_DEVICE.

Cc: stable@vger.kernel.org # v6.13+
Tested-by: Han Gao <gaohan@iscas.ac.cn> # SG2044
Signed-off-by: Vivian Wang <wangruikang@iscas.ac.cn>
Link: https://patch.msgid.link/20260309-riscv-sparsemem-vmemmap-limits-v1-2-f40efe18e3cd@iscas.ac.cn
Signed-off-by: Paul Walmsley <pjw@kernel.org>