set_page_dirty variants

[Matthew Wilcox <willy@infradead.org>]

We currently have three near-identical implementations of the
set_page_dirty address_space op:

__set_page_dirty_no_writeback added 2007 by Ken Chen (767193253bba)
(return value fixed by Bob Liu in 2011 (c3f0da631539))
anon_set_page_dirty added 2009 by Peter Zijlstra (d3a9262e59f7)
noop_set_page_dirty added 2018 by Dan Williams (f44c77630d26)

I persuaded Mike to remove hugetlbfs_set_page_dirty and
Daniel Vetter to remove fb_deferred_io_set_page_dirty (in -next)
so we're down from five to three.

I'd like to get it down to zero.  After all, the !mapping case in
set_page_dirty() is exactly what we want.  So is there a problem
with doing this?

+++ b/mm/page-writeback.c
@@ -2562 +2562 @@ int set_page_dirty(struct page *page)
-       if (likely(mapping)) {
+       if (likely(mapping && mapping_can_writeback(mapping))) {

But then I noticed that we have both mapping_can_writeback()
and mapping_use_writeback_tags(), and I'm no longer sure
which one to use.  Also, why don't we mirror the results of
inode_to_bdi(mapping->host)->capabilities & BDI_CAP_WRITEBACK into
a mapping->flags & AS_something bit?  We have lots available, and
inode_to_bdi seems relatively complicated to be a static inline that
gets evaluated every time we call
pagecache_get_page(FGP_CREAT | FGP_WRITE).