iommu: Replace device_lock() with group->mutex

device_lock() was used in iommu_group_store_type() to prevent the
devices in an iommu group from being attached by any device driver.
On the other hand, in order to avoid lock race between group->mutex
and device_lock(), it limited the usage scenario to the singleton
groups.

We already have the DMA ownership scheme to avoid driver attachment
and group->mutex ensures that device ops are always valid, there's
no need for device_lock() anymore. Remove device_lock() and the
singleton group limitation.

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20230322064956.263419-6-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>