spi: spidev: Align buffers for DMA

Simply copying all xfers from userspace into one bounce buffer causes
alignment problems if the SPI controller uses DMA.

Ensure that all transfer data blocks within the rx and tx bounce buffers
are aligned for DMA (according to ARCH_KMALLOC_MINALIGN).

Alignment may increase the usage of the bounce buffers. In some cases,
the buffers may need to be increased using the "bufsiz" module
parameter.

Signed-off-by: Christian Eggers <ceggers@arri.de>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200728100832.24788-1-ceggers@arri.de


Signed-off-by: Mark Brown <broonie@kernel.org>