mm/swap: convert find_get_incore_page to use folios

Eliminates a use of FGP_HEAD and saves 35 bytes of text.

Link: https://lkml.kernel.org/r/20221019183332.2802139-3-willy@infradead.org


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