mm: rename PG_mappedtodisk to PG_owner_2

This flag has similar constraints to PG_owner_priv_1 -- it is ignored by
core code, and is entirely for the use of the code which allocated the
folio.  Since the pagecache does not use it, individual filesystems can
use it.  The bufferhead code does use it, so filesystems which use the
buffer cache must not use it for another purpose.

Link: https://lkml.kernel.org/r/20240821193445.2294269-10-willy@infradead.org


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