ptr_ring: try vmalloc() when kmalloc() fails

This patch switch to use kvmalloc_array() for using a vmalloc()
fallback to help in case kmalloc() fails.

Reported-by:  <syzbot+e4d4f9ddd4295539735d@syzkaller.appspotmail.com>
Fixes: 2e0ab8ca ("ptr_ring: array based FIFO for pointers")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>