dmar: use list_for_each_entry_safe() in dmar_dev_scope_init()

In dmar_dev_scope_init(), functions called under for_each_drhd_unit()/
for_each_rmrr_units() can delete the list entry under some error conditions.

So we should use list_for_each_entry_safe() for safe traversal.

Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Acked-by: Yinghai Lu <yhlu.kernel@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>