dma-iommu: use static-key to minimize the impact in the fast-path

Let's move out the is_kdump_kernel() check from iommu_dma_deferred_attach()
to iommu_dma_init(), and use the static-key in the fast-path to minimize
the impact in the normal case.

Co-developed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Lianbo Jiang <lijiang@redhat.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20210126115337.20068-2-lijiang@redhat.com


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