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

In the Linux kernel, the following vulnerability has been resolved:

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]