mm/huge_memory: add a common exit path to zap_huge_pmd()

Other than when we acquire the PTL, we always need to unlock the PTL, and
optionally need to flush on exit.

The code is currently very duplicated in this respect, so default
flush_needed to false, set it true in the case in which it's required,
then share the same logic for all exit paths.

This also makes flush_needed make more sense as a function-scope value (we
don't need to flush for the PFN map/mixed map, zero huge, error cases for
instance).

Link: https://lkml.kernel.org/r/6b281d8ed972dff0e89bdcbdd810c96c7ae8c9dc.1774029655.git.ljs@kernel.org
Signed-off-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Suren Baghdasaryan <surenb@google.com>
Cc: Barry Song <baohua@kernel.org>
Cc: David Hildenbrand <david@kernel.org>
Cc: Dev Jain <dev.jain@arm.com>
Cc: Lance Yang <lance.yang@linux.dev>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Cc: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>