fs: convert block_write_full_page to block_write_full_folio

Convert the function to be compatible with writepage_t so that it can be
passed to write_cache_pages() by blkdev.  This removes a call to
compound_head().  We can also remove the function export as both callers
are built-in.

Link: https://lkml.kernel.org/r/20231215200245.748418-14-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>