spi: fix null pointer dereference within spi_sync

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

spi: fix null pointer dereference within spi_sync

If spi_sync() is called with the non-empty queue and the same spi_message
is then reused, the complete callback for the message remains set while
the context is cleared, leading to a null pointer dereference when the
callback is invoked from spi_finalize_current_message().

With function inlining disabled, the call stack might look like this:

_raw_spin_lock_irqsave from complete_with_flags+0x18/0x58
complete_with_flags from spi_complete+0x8/0xc
spi_complete from spi_finalize_current_message+0xec/0x184
spi_finalize_current_message from spi_transfer_one_message+0x2a8/0x474
spi_transfer_one_message from __spi_pump_transfer_message+0x104/0x230
__spi_pump_transfer_message from __spi_transfer_message_noqueue+0x30/0xc4
__spi_transfer_message_noqueue from __spi_sync+0x204/0x248
__spi_sync from spi_sync+0x24/0x3c
spi_sync from mcp251xfd_regmap_crc_read+0x124/0x28c [mcp251xfd]
mcp251xfd_regmap_crc_read [mcp251xfd] from _regmap_raw_read+0xf8/0x154
_regmap_raw_read from _regmap_bus_read+0x44/0x70
_regmap_bus_read from _regmap_read+0x60/0xd8
_regmap_read from regmap_read+0x3c/0x5c
regmap_read from mcp251xfd_alloc_can_err_skb+0x1c/0x54 [mcp251xfd]
mcp251xfd_alloc_can_err_skb [mcp251xfd] from mcp251xfd_irq+0x194/0xe70 [mcp251xfd]
mcp251xfd_irq [mcp251xfd] from irq_thread_fn+0x1c/0x78
irq_thread_fn from irq_thread+0x118/0x1f4
irq_thread from kthread+0xd8/0xf4
kthread from ret_from_fork+0x14/0x28

Fix this by also setting message->complete to NULL when the transfer is
complete.