mm: add folio_get_nontail_page()

Patch series "Convert a couple migrate functions to use folios", v2.

This patchset introduces folio_movable_ops() and converts 3 functions in
mm/migrate.c to use folios.  It also introduces folio_get_nontail_page()
for folio conversions which may want to distinguish between head and tail
pages.


This patch (of 4):

folio_get_nontail_page() returns the folio associated with a head page. 
This is necessary for folio conversions where the behavior of that
function differs between head pages and tail pages.

Link: https://lkml.kernel.org/r/20230130214352.40538-1-vishal.moola@gmail.com
Link: https://lkml.kernel.org/r/20230130214352.40538-2-vishal.moola@gmail.com


Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>