iommu: remove DOMAIN_ATTR_NESTING

Use an explicit enable_nesting method instead.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Will Deacon <will@kernel.org>
Acked-by: Li Yang <leoyang.li@nxp.com>
Link: https://lore.kernel.org/r/20210401155256.298656-17-hch@lst.de


Signed-off-by: Joerg Roedel <jroedel@suse.de>