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>

Author: XiaoWang1772

arch/riscv/lib/uaccess.S

@@ -44,7 +44,7 @@ SYM_FUNC_START(fallback_scalar_usercopy)
 	 * Use byte copy only if too small.
 	 * SZREG holds 4 for RV32 and 8 for RV64
 	 */
-	li	a3, 9*SZREG /* size must be larger than size in word_copy */
+	li	a3, 9*SZREG-1 /* size must >= (word_copy stride + SZREG-1) */
 	bltu	a2, a3, .Lbyte_copy_tail
 
 	/*