s390/vfio-ap: Make mdev_types not look like a fake flex array

s390/vfio-ap: Make mdev_types not look like a fake flex array

The vfio-ap driver and the vfio parent device provided by it
(matrix_dev) support just a single mdev_type, and this is not likely to
change any time soon.  Despite that matrix_dev->mdev_types started out
as a C99 flexible array presumably as a typo, and since the typo messed
up the allocation, commit e2c8cee9f489 ("s390/vfio-ap: Fix memory
allocation for mdev_types array") changed it to an array of size 1. And
to make things worse mdev_types happens to be the last member of struct
ap_matrix_dev.

Now the problem with that is that before C99 the usual way to get
something similar to a flexible array member was to use a trailing array of
size 0 or 1. This is what I called fake flex array. For a while now the
community is trying to get rid of fake flex arrays. And while mdev_types
is not a fake flex array but an array of size one (to match the mdev
interfaces nicer), it can easily be and was mistaken for a fake flex
array.

So, let us make mdev_types a pointer to struct mdev_type and pass in the
address of that pointer as the 4th formal parameter of
mdev_register_parent().

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Reviewed-by: Anthony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Eric Farman <farman@linux.ibm.com>
Tested-by: Anthony Krowiak <akrowiak@linux.ibm.com>
Link: https://lore.kernel.org/r/20250217100614.3043620-2-pasic@linux.ibm.com
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>