iommu/vt-d: Use bitmap_zalloc() when applicable

'iommu->domain_ids' is a bitmap. So use 'bitmap_zalloc()' to simplify code
and improve the semantic.

Also change the corresponding 'kfree()' into 'bitmap_free()' to keep
consistency.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/cb7a3e0a8d522447a06298a4f244c3df072f948b.1635018498.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/20211217083817.1745419-2-baolu.lu@linux.intel.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>