iommu/vt-d: Remove misleading "domain 0" test from domain_exit()

The "Domain 0 is reserved, so dont process it" comment suggests that a NULL
pointer corresponds to domain 0.  I don't think that's true, and in any
case, every caller supplies a non-NULL domain pointer that has already been
dereferenced, so the test is unnecessary.

Remove the test for a null "domain" pointer.  No functional change
intended.

This null pointer check was added by 5e98c4b1 ("Allocation and free
functions of virtual machine domain").

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>