bcachefs: stop using write_cache_pages

bcachefs: stop using write_cache_pages

Stop using the obsolete write_cache_pages and use writeback_iter directly.
This basically just open codes write_cache_pages without the indirect
call, but there's probably ways to structure the code even nicer as a
follow on.

Link: https://lkml.kernel.org/r/20250818061017.1526853-3-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: David Hildenbrand <david@redhat.com>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>