mips: mm: convert __flush_dcache_pages() to __flush_dcache_folio_pages()

mips: mm: convert __flush_dcache_pages() to __flush_dcache_folio_pages()

Let's make it clearer that we are operating within a single folio by
providing both the folio and the page.

This implies that for flush_dcache_folio() we'll now avoid one more
page->folio lookup, and that we can safely drop the "nth_page" usage.

While at it, drop the "extern" from the function declaration.

Link: https://lkml.kernel.org/r/20250901150359.867252-22-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Cc: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>