powerpc/iommu: Use bitmap library

- Caluculate the bitmap size with BITS_TO_LONGS()
 - Use bitmap_empty() to verify that all bits are cleared

This also includes a printk to pr_warn() conversion.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linuxppc-dev@lists.ozlabs.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>