vfio/nvgrace-gpu: Add Blackwell-Next GPU readiness check via CXL DVSEC

Add a CXL DVSEC-based readiness check for Blackwell-Next GPUs alongside
the existing legacy BAR0 polling path. The CXL Device DVSEC offset is
discovered at probe time. Probe, fault and read/write paths then branch
on that to use either the legacy BAR0 polling or the CXL DVSEC polling.

The CXL path polls Memory_Active, requiring MEM_INFO_VALID within 1s and
MEM_ACTIVE within Memory_Active_Timeout (up to 256s) as per CXL spec r4.0
sec 8.1.3.8.2. Given the long worst-case wait, the CXL poll runs outside
memory_lock with only a quick readiness check is done under the lock.

The poll loops sleep with schedule_timeout_killable() and return -EINTR
on a fatal signal. This avoids hung-task panics during the long
uninterruptible wait. Extend this to the legacy based wait as well for
improvement.

In the fault handler the wait runs locklessly before memory_lock. If a
reset races in, the in-lock recheck returns -EAGAIN and the wait is
retried rather than returning a spurious VM_FAULT_SIGBUS.

Add PCI_DVSEC_CXL_MEM_ACTIVE_TIMEOUT to pci_regs.h for the timeout field.

Cc: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Cc: Kevin Tian <kevin.tian@intel.com>
Suggested-by: Alex Williamson <alex@shazbot.org>
Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20260602063015.3915-1-ankita@nvidia.com
Signed-off-by: Alex Williamson <alex@shazbot.org>

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