mm: Refector release_pages()

In preparation for implementing folios_put_refs() in the next patch,
refactor release_pages() into a set of helper functions, which can be
reused. The primary difference between release_pages() and
folios_put_refs() is how they iterate over the set of folios. The
per-folio actions are identical.

No functional change intended.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>