riscv: uaccess: Relax the threshold for fast path

The bytes copy for unaligned head would cover at most SZREG-1 bytes, so
it's better to set the threshold as >= (SZREG-1 + word_copy stride size)
which equals to 9*SZREG-1.

Signed-off-by: Xiao Wang <xiao.w.wang@intel.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20240313091929.4029960-1-xiao.w.wang@intel.com


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