buffer: replace obvious uses of b_page with b_folio

These cases just check if it's NULL, or use b_page to get to the page's
address space.  They are assumptions that b_page never points to a tail
page.

Link: https://lkml.kernel.org/r/20221215214402.3522366-3-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/buffer.c

@@ -321,7 +321,7 @@ static void end_buffer_async_read_io(struct buffer_head *bh, int uptodate)
 {
 	/* Decrypt if needed */
 	if (uptodate &&
-	    fscrypt_inode_uses_fs_layer_crypto(bh->b_page->mapping->host)) {
+	    fscrypt_inode_uses_fs_layer_crypto(bh->b_folio->mapping->host)) {
 		struct decrypt_bh_ctx *ctx = kmalloc(sizeof(*ctx), GFP_ATOMIC);
 
 		if (ctx) {
@@ -570,7 +570,7 @@ void write_boundary_block(struct block_device *bdev,
 void mark_buffer_dirty_inode(struct buffer_head *bh, struct inode *inode)
 {
 	struct address_space *mapping = inode->i_mapping;
-	struct address_space *buffer_mapping = bh->b_page->mapping;
+	struct address_space *buffer_mapping = bh->b_folio->mapping;
 
 	mark_buffer_dirty(bh);
 	if (!mapping->private_data) {
@@ -1073,7 +1073,7 @@ __getblk_slow(struct block_device *bdev, sector_t block,
  * and then attach the address_space's inode to its superblock's dirty
  * inode list.
  *
- * mark_buffer_dirty() is atomic.  It takes bh->b_page->mapping->private_lock,
+ * mark_buffer_dirty() is atomic.  It takes bh->b_folio->mapping->private_lock,
  * i_pages lock and mapping->host->i_lock.
  */
 void mark_buffer_dirty(struct buffer_head *bh)
@@ -1117,8 +1117,8 @@ void mark_buffer_write_io_error(struct buffer_head *bh)
 
 	set_buffer_write_io_error(bh);
 	/* FIXME: do we need to set this in both places? */
-	if (bh->b_page && bh->b_page->mapping)
-		mapping_set_error(bh->b_page->mapping, -EIO);
+	if (bh->b_folio && bh->b_folio->mapping)
+		mapping_set_error(bh->b_folio->mapping, -EIO);
 	if (bh->b_assoc_map)
 		mapping_set_error(bh->b_assoc_map, -EIO);
 	rcu_read_lock();
@@ -1154,7 +1154,7 @@ void __bforget(struct buffer_head *bh)
 {
 	clear_buffer_dirty(bh);
 	if (bh->b_assoc_map) {
-		struct address_space *buffer_mapping = bh->b_page->mapping;
+		struct address_space *buffer_mapping = bh->b_folio->mapping;
 
 		spin_lock(&buffer_mapping->private_lock);
 		list_del_init(&bh->b_assoc_buffers);