misc: pci_endpoint_test: Use memcpy_toio()/memcpy_fromio() for BAR tests

misc: pci_endpoint_test: Use memcpy_toio()/memcpy_fromio() for BAR tests

The current code uses writel()/readl(), which has an implicit memory
barrier for every single readl()/writel().

Additionally, reading 4 bytes at a time over the PCI bus is not really
optimal, considering that this code is running in an ioctl handler.

Use memcpy_toio()/memcpy_fromio() for BAR tests.

Before patch with a 4MB BAR:

  $ time /usr/bin/pcitest -b 1
  BAR1:           OKAY
  real    0m 1.56s

After patch with a 4MB BAR:

  $ time /usr/bin/pcitest -b 1
  BAR1:           OKAY
  real    0m 0.54s

Link: https://lore.kernel.org/linux-pci/20240322164139.678228-1-cassel@kernel.org
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>