buffer: Use folio_next_pos()

This is one instruction more efficient than open-coding folio_pos() +
folio_size().  It's the equivalent of (x + y) << z rather than
x << z + y << z.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://patch.msgid.link/20251024170822.1427218-4-willy@infradead.org
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Matthew Wilcox (Oracle)

fs/buffer.c

@@ -2732,7 +2732,7 @@ int block_write_full_folio(struct folio *folio, struct writeback_control *wbc,
 	loff_t i_size = i_size_read(inode);
 
 	/* Is the folio fully inside i_size? */
-	if (folio_pos(folio) + folio_size(folio) <= i_size)
+	if (folio_next_pos(folio) <= i_size)
 		return __block_write_full_folio(inode, folio, get_block, wbc);
 
 	/* Is the folio fully outside i_size? (truncate in progress) */