proc: convert smaps_account() to use a folio

Replace seven calls to compound_head() with one.

Link: https://lkml.kernel.org/r/20240402201252.917342-3-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>