mm: move page->deferred_list to folio->_deferred_list

Remove the entire block of definitions for the second tail page, and add
the deferred list to the struct folio.  This actually moves _deferred_list
to a different offset in struct folio because I don't see a need to
include the padding.

This lets us use list_for_each_entry_safe() in deferred_split_scan()
and avoid a number of calls to compound_head().

Link: https://lkml.kernel.org/r/20230111142915.1001531-25-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>