userfaultfd: fix a crash in UFFDIO_MOVE when PMD is a migration entry

When UFFDIO_MOVE encounters a migration PMD entry, it proceeds with
obtaining a folio and accessing it even though the entry is swp_entry_t. 
Add the missing check and let split_huge_pmd() handle migration entries. 
While at it also remove unnecessary folio check.

[surenb@google.com: remove extra folio check, per David]
  Link: https://lkml.kernel.org/r/20250807200418.1963585-1-surenb@google.com
Link: https://lkml.kernel.org/r/20250806220022.926763-1-surenb@google.com


Fixes: adef4406 ("userfaultfd: UFFDIO_MOVE uABI")
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Reported-by:  <syzbot+b446dbe27035ef6bd6c2@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/68794b5c.a70a0220.693ce.0050.GAE@google.com/


Reviewed-by: Peter Xu <peterx@redhat.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Lokesh Gidra <lokeshgidra@google.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>