iomap: Mark read blocks uptodate in write_begin

When bringing (portions of) a page uptodate, we were marking blocks that
were zeroed as being uptodate, but not blocks that were read from storage.

Like the previous commit, this problem was found with generic/127 and
a kernel which failed readahead I/Os.  This bug causes writes to be
silently lost when working with flaky storage.

Fixes: 9dc55f13 ("iomap: add support for sub-pagesize buffered I/O without buffer heads")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>

fs/iomap/buffered-io.c

@@ -574,7 +574,6 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
 	loff_t block_start = pos & ~(block_size - 1);
 	loff_t block_end = (pos + len + block_size - 1) & ~(block_size - 1);
 	unsigned from = offset_in_page(pos), to = from + len, poff, plen;
-	int status;
 
 	if (PageUptodate(page))
 		return 0;
@@ -595,14 +594,13 @@ __iomap_write_begin(struct inode *inode, loff_t pos, unsigned len, int flags,
 			if (WARN_ON_ONCE(flags & IOMAP_WRITE_F_UNSHARE))
 				return -EIO;
 			zero_user_segments(page, poff, from, to, poff + plen);
-			iomap_set_range_uptodate(page, poff, plen);
-			continue;
+		} else {
+			int status = iomap_read_page_sync(block_start, page,
+					poff, plen, srcmap);
+			if (status)
+				return status;
 		}
-
-		status = iomap_read_page_sync(block_start, page, poff, plen,
-				srcmap);
-		if (status)
-			return status;
+		iomap_set_range_uptodate(page, poff, plen);
 	} while ((block_start += plen) < block_end);
 
 	return 0;