buffer: add b_folio as an alias of b_page

buffer: add b_folio as an alias of b_page

Patch series "Start converting buffer_heads to use folios".

I was hoping that filesystems would convert from buffer_heads to iomap,
but that's not happening particularly quickly.  So the buffer_head
infrastructure needs to be converted from being page-based to being
folio-based.

This patch (of 12):

Buffer heads point to the allocation (ie the folio), not the page.  This
is currently the same thing for all filesystems that use buffer heads, so
this is a safe transitional step.

Link: https://lkml.kernel.org/r/20221215214402.3522366-1-willy@infradead.org
Link: https://lkml.kernel.org/r/20221215214402.3522366-2-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>