Mark the functions check_zero_address() and dmar_get_fault_reason() as
static in dmar.c because they are not used outside this file.
This eliminates the following warnings in dmar.c:
drivers/iommu/dmar.c:491:12: warning: no previous prototype for ‘check_zero_address’ [-Wmissing-prototypes]
drivers/iommu/dmar.c:1116:13: warning: no previous prototype for ‘dmar_get_fault_reason’ [-Wmissing-prototypes]