riscv: mm: Fixup no5lvl failure when vaddr is invalid

riscv: mm: Fixup no5lvl failure when vaddr is invalid

Unlike no4lvl, no5lvl still continues to detect satp, which
requires va=pa mapping. When pa=0x800000000000, no5lvl
would fail in Sv48 mode due to an illegal VA value of
0x800000000000.

So, prevent detecting the satp flow for no5lvl, when
vaddr is invalid. Add the is_vaddr_valid() function for
checking.

Fixes: 26e7aacb83df ("riscv: Allow to downgrade paging mode from the command line")
Cc: Alexandre Ghiti <alexghiti@rivosinc.com>
Cc: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Guo Ren (Alibaba DAMO Academy) <guoren@kernel.org>
Tested-by: Fangyu Yu <fangyu.yu@linux.alibaba.com>
Link: https://patch.msgid.link/20260125055212.433163-1-guoren@kernel.org
[pjw@kernel.org: cleaned up commit message]
Signed-off-by: Paul Walmsley <pjw@kernel.org>