ext4: 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/20251024170822.1427218-5-willy@infradead.org
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: linux-ext4@vger.kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Matthew Wilcox (Oracle)

fs/ext4/inode.c

@@ -1319,8 +1319,8 @@ static int ext4_write_begin(const struct kiocb *iocb,
 	if (IS_ERR(folio))
 		return PTR_ERR(folio);
 
-	if (pos + len > folio_pos(folio) + folio_size(folio))
-		len = folio_pos(folio) + folio_size(folio) - pos;
+	if (len > folio_next_pos(folio) - pos)
+		len = folio_next_pos(folio) - pos;
 
 	from = offset_in_folio(folio, pos);
 	to = from + len;
@@ -2704,7 +2704,7 @@ static int mpage_prepare_extent_to_map(struct mpage_da_data *mpd)
 
 			if (mpd->map.m_len == 0)
 				mpd->start_pos = folio_pos(folio);
-			mpd->next_pos = folio_pos(folio) + folio_size(folio);
+			mpd->next_pos = folio_next_pos(folio);
 			/*
 			 * Writeout when we cannot modify metadata is simple.
 			 * Just submit the page. For data=journal mode we
@@ -3146,8 +3146,8 @@ static int ext4_da_write_begin(const struct kiocb *iocb,
 	if (IS_ERR(folio))
 		return PTR_ERR(folio);
 
-	if (pos + len > folio_pos(folio) + folio_size(folio))
-		len = folio_pos(folio) + folio_size(folio) - pos;
+	if (len > folio_next_pos(folio) - pos)
+		len = folio_next_pos(folio) - pos;
 
 	ret = ext4_block_write_begin(NULL, folio, pos, len,
 				     ext4_da_get_block_prep);