mm: remove page_ref_sub_return()

With all callers converted to folios, we can act directly on
folio->_refcount.

Link: https://lkml.kernel.org/r/20240424191914.361554-5-willy@infradead.org


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