spi: tegra210-quad: Protect curr_xfer check in IRQ handler

spi: tegra210-quad: Protect curr_xfer check in IRQ handler

Now that all other accesses to curr_xfer are done under the lock,
protect the curr_xfer NULL check in tegra_qspi_isr_thread() with the
spinlock. Without this protection, the following race can occur:

  CPU0 (ISR thread)              CPU1 (timeout path)
  ----------------               -------------------
  if (!tqspi->curr_xfer)
    // sees non-NULL
                                 spin_lock()
                                 tqspi->curr_xfer = NULL
                                 spin_unlock()
  handle_*_xfer()
    spin_lock()
    t = tqspi->curr_xfer  // NULL!
    ... t->len ...        // NULL dereference!

With this patch, all curr_xfer accesses are now properly synchronized.

Although all accesses to curr_xfer are done under the lock, in
tegra_qspi_isr_thread() it checks for NULL, releases the lock and
reacquires it later in handle_cpu_based_xfer()/handle_dma_based_xfer().
There is a potential for an update in between, which could cause a NULL
pointer dereference.

To handle this, add a NULL check inside the handlers after acquiring
the lock. This ensures that if the timeout path has already cleared
curr_xfer, the handler will safely return without dereferencing the
NULL pointer.

Fixes: b4e002d8a7ce ("spi: tegra210-quad: Fix timeout handling")
Signed-off-by: Breno Leitao <leitao@debian.org>
Tested-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Link: https://patch.msgid.link/20260126-tegra_xfer-v2-6-6d2115e4f387@debian.org
Signed-off-by: Mark Brown <broonie@kernel.org>