VT-d: eliminate a mis-use of pcidevs_lock

VT-d: eliminate a mis-use of pcidevs_lock

dma_pte_clear_one() shouldn't acquire this global lock for the purpose
of processing a per-domain list. Furthermore the function a few lines
earlier has a comment stating that acquiring pcidevs_lock isn't
necessary here (whether that's really correct is another question).

Use the domain's mappin_lock instead to protect the mapped_rmrrs list.
Fold domain_rmrr_mapped() into its sole caller so that the otherwise
implicit dependency on pcidevs_lock there becomes more obvious (see
the comment there).

Signed-off-by: Jan Beulich <jbeulich@suse.com>