spi: fix null pointer dereference within spi_sync

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.

Fixes: ae7d2346dc89 ("spi: Don't use the message queue if possible in spi_sync")
Signed-off-by: Mans Rullgard <mans@mansr.com>
Link: https://lore.kernel.org/r/20240430182705.13019-1-mans@mansr.com
Signed-off-by: Mark Brown <broonie@kernel.org>