spi: spidev: Convert buf pointers for 32-bit compat SPI_IOC_MESSAGE(n)

The SPI_IOC_MESSAGE(n) ioctl commands' argument points to an array of n
struct spi_ioc_transfer elements.  The spidev's compat_ioctl handler
just converts this pointer and passes it on to the unlocked_ioctl
handler to process it.

The tx_buf and rx_buf members of struct spi_ioc_transfer are of type
__u64 and hold pointer values.  A 32-bit userspace application running
in a 64-bit kernel might not have widened the 32-bit pointers correctly
for the kernel.  The application might have sign-extended the pointer to
when the kernel expects it to be zero-extended, or vice versa, leading
to an -EFAULT being returned by spidev_message() if the widened pointer
is invalid.

Handle the SPI_IOC_MESSAGE(n) ioctl commands specially in the
compat_ioctl handler, calling new function spidev_compat_ioctl_message()
to handle them.  This processes them in the same way as the
unlocked_ioctl handler except that it uses compat_ptr() to convert the
tx_buf and rx_buf members of each struct spi_ioc_transfer element.

To save code, factor out part of the unlocked_ioctl handler into a new
function spidev_get_ioc_message().  This checks the ioctl command code
is a valid SPI_IOC_MESSAGE(n), determines n and copies the array of n
struct spi_ioc_transfer elements from userspace into dynamically
allocated memory, returning either a pointer to the memory, an
ERR_PTR(-err) value, or NULL (for SPI_IOC_MESSAGE(0)).

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Mark Brown <broonie@kernel.org>