spi: Fix DMA mapping ownership on partial map failure

In the Linux kernel, the following vulnerability has been resolved:

spi: Fix DMA mapping ownership on partial map failure

If RX mapping fails after TX mapping succeeds, __spi_map_msg() unmaps
TX but leaves tx_sg_mapped set. If TX mapping fails on a later
transfer, mappings created for earlier transfers remain active.

In both cases, cur_{tx,rx}_dma_dev have not yet been updated because they
are assigned only after every transfer has been mapped. The subsequent
spi_unmap_msg() may therefore unmap the TX mapping again or release
earlier mappings using a NULL or stale device. Using a NULL device can
trigger an oops. An empty SG table does not prevent the NULL dereference
because dma_unmap_sg_attrs() accesses the device before checking the
entry count.

Publish both mapping devices before mapping starts and unwind all
failures through __spi_unmap_msg(). This clears the mapping flags and
releases each mapping once with the device that created it.

Publishing the devices before the loop also refreshes them when no
transfer needs mapping. No mapping flag is set in that case, so current
users do not use the pointers as mapping owners.