vfio iommu: Add dma available capability

[ Upstream commit 7d6e1329 ]

The following functional changes were needed for backport:
- vfio_iommu_type1_get_info doesn't exist, call
  vfio_iommu_dma_avail_build_caps from vfio_iommu_type1_ioctl.
- As further fallout from this, vfio_iommu_dma_avail_build_caps must
  acquire and release the iommu mutex lock.  To do so, the return value is
  stored in a local variable as in vfio_iommu_iova_build_caps.

Upstream commit description:
Commit 49285593 ("vfio/type1: Limit DMA mappings per container")
added the ability to limit the number of memory backed DMA mappings.
However on s390x, when lazy mapping is in use, we use a very large
number of concurrent mappings.  Let's provide the current allowable
number of DMA mappings to userspace via the IOMMU info chain so that
userspace can take appropriate mitigation.

Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>