spi: spidev: Hold spi_lock over all defererences of spi in release()

We use the spi_lock spinlock to protect against races between the device
being removed and file operations on the spidev.  This means that in the
removal path all references to the device need to be done under lock as
in removal we dropping references to the device.

Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>