iommu/amd: Cleanup error handling in do_fault()

Get rid of the three error paths that look the same and move
error handling to a single place.

Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Acked-By: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>