CONFIG_FAIL_IOMMU is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

Fault-injection capability for IOMMU


Linux Kernel Configuration
└─>Fault-injection capability for IOMMU
In linux kernel since version 3.1 (release Date: 2011-10-24)  
Provide fault-injection capability for IOMMU. Each device can
be selectively enabled via the fail_iommu property.

If you are unsure, say N.