mm/migrate_device: remove dead migration entry check in migrate_vma_collect_huge_pmd()

mm/migrate_device: remove dead migration entry check in migrate_vma_collect_huge_pmd()

The softleaf_is_migration() check is unreachable as entries that are not
device_private are filtered out.  Similarly, the PTE-level equivalent in
migrate_vma_collect_pmd() skips migration entries.

This dead branch also contained a double spin_unlock(ptl) bug.

Link: https://lore.kernel.org/20260212014611.416695-1-dave@stgolabs.net
Fixes: a30b48bf1b244 ("mm/migrate_device: implement THP migration of zone device pages")
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Suggested-by: Matthew Brost <matthew.brost@intel.com>
Reviewed-by: Alistair Popple <apopple@nvidia.com>
Acked-by: Balbir Singh <balbirs@nvidia.com>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Cc: Byungchul Park <byungchul@sk.com>
Cc: Gregory Price <gourry@gourry.net>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Joshua Hahn <joshua.hahnjy@gmail.com>
Cc: Mathew Brost <matthew.brost@intel.com>
Cc: Rakie Kim <rakie.kim@sk.com>
Cc: Ying Huang <ying.huang@linux.alibaba.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>