s390/vfio-ap: Ignore duplicate link requests in vfio_ap_mdev_link_queue

vfio_ap_mdev_link_queue is changed to detect if a matrix_mdev has
already linked the given queue. If so, it bails out.

Signed-off-by: "Jason J. Herne" <jjherne@linux.ibm.com>
Reviewed-by: Tony Krowiak <akrowiak@linux.ibm.com>
Link: https://lore.kernel.org/r/20240415152555.13152-4-jjherne@linux.ibm.com
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>