riscv: Fix missing __free_pages() in check_vector_unaligned_access()

The locally allocated pages are never freed up, so add the corresponding
__free_pages().

Fixes: e7c9d66e ("RISC-V: Report vector unaligned access speed hwprobe")
Link: https://lore.kernel.org/r/20250228090613.345309-1-alexghiti@rivosinc.com


Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>