iommufd/selftest: Fix mock_dev_num bug

Syzkaller reported the following bug:
  sysfs: cannot create duplicate filename '/devices/iommufd_mock4'

  Call Trace:
    sysfs_warn_dup+0x71/0x90
    sysfs_create_dir_ns+0x1ee/0x260
    ? sysfs_create_mount_point+0x80/0x80
    ? spin_bug+0x1d0/0x1d0
    ? do_raw_spin_unlock+0x54/0x220
    kobject_add_internal+0x221/0x970
    kobject_add+0x11c/0x1e0
    ? lockdep_hardirqs_on_prepare+0x273/0x3e0
    ? kset_create_and_add+0x160/0x160
    ? kobject_put+0x5d/0x390
    ? bus_get_dev_root+0x4a/0x60
    ? kobject_put+0x5d/0x390
    device_add+0x1d5/0x1550
    ? __fw_devlink_link_to_consumers.isra.0+0x1f0/0x1f0
    ? __init_waitqueue_head+0xcb/0x150
    iommufd_test+0x462/0x3b60
    ? lock_release+0x1fe/0x640
    ? __might_fault+0x117/0x170
    ? reacquire_held_locks+0x4b0/0x4b0
    ? iommufd_selftest_destroy+0xd0/0xd0
    ? __might_fault+0xbe/0x170
    iommufd_fops_ioctl+0x256/0x350
    ? iommufd_option+0x180/0x180
    ? __lock_acquire+0x1755/0x45f0
    __x64_sys_ioctl+0xa13/0x1640

The bug is triggered when Syzkaller created multiple mock devices but
didn't destroy them in the same sequence, messing up the mock_dev_num
counter. Replace the atomic with an mock_dev_ida.

Cc: stable@vger.kernel.org
Fixes: 23a1b46f

 ("iommufd/selftest: Make the mock iommu driver into a real driver")
Link: https://lore.kernel.org/r/5af41d5af6d5c013cc51de01427abb8141b3587e.1708636627.git.nicolinc@nvidia.com
Reported-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>