powerpc/iommu: fix memory leak with using debugfs_lookup()

In the Linux kernel, the following vulnerability has been resolved:

powerpc/iommu: fix memory leak with using debugfs_lookup()

When calling debugfs_lookup() the result must have dput() called on it,

otherwise the memory will leak over time. To make things simpler, just

call debugfs_lookup_and_remove() instead which handles all of the logic

at once.