mm/vmscan: Free non-shmem folios without splitting them

We have to allocate memory in order to split a file-backed folio, so
it's not a good idea to split them in the memory freeing path.  It also
doesn't work for XFS because pages have an extra reference count from
page_has_private() and split_huge_page() expects that reference to have
already been removed.  Unfortunately, we still have to split shmem THPs
because we can't handle swapping out an entire THP yet.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>