riscv: uaccess: Allow the last potential unrolled copy

When the dst buffer pointer points to the last accessible aligned addr, we
could still run another iteration of unrolled copy.

Signed-off-by: Xiao Wang <xiao.w.wang@intel.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240313103334.4036554-1-xiao.w.wang@intel.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>