vfio/pci: WARN_ON driver_override kasprintf failure

vfio/pci: WARN_ON driver_override kasprintf failure

kasprintf() returns a pointer to dynamically allocated memory
which can be NULL upon failure.

This is a blocking notifier callback, so errno isn't a proper return
value. Use WARN_ON to small allocation failures.

Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Link: https://lore.kernel.org/r/20240115063434.20278-1-chentao@kylinos.cn
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>