spi: core: Fix deadlock when sending messages

The function __spi_pump_messages() is called by spi_pump_messages() and
__spi_sync(). The function __spi_sync() has an argument 'bus_locked'
that indicates if it is called with the SPI bus mutex held or not. If
'bus_locked' is false then __spi_sync() will acquire the mutex itself.

Commit 556351f1 ("spi: introduce accelerated read support for spi
flash devices") made a change to acquire the SPI bus mutex within
__spi_pump_messages(). However, this change did not check to see if the
mutex is already held. If __spi_sync() is called with the mutex held
(ie. 'bus_locked' is true), then a deadlock occurs when
__spi_pump_messages() is called.

Fix this deadlock by passing the 'bus_locked' state from __spi_sync() to
__spi_pump_messages() and only acquire the mutex if not already held. In
the case where __spi_pump_messages() is called from spi_pump_messages()
it is assumed that the mutex is not held and so call
__spi_pump_messages() with 'bus_locked' set to false. Finally, move the
unlocking of the mutex to the end of the __spi_pump_messages() function
to simplify the code and only call cond_resched() if there are no
errors.

Fixes: 556351f1 ("spi: introduce accelerated read support for spi flash devices")

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>