squashfs: fix extending readahead beyond end of file

The readahead code will try to extend readahead to the entire size of the
Squashfs data block.

But, it didn't take into account that the last block at the end of the
file may not be a whole block.  In this case, the code would extend
readahead to beyond the end of the file, leaving trailing pages.

Fix this by only requesting the expected number of pages.

Link: https://lkml.kernel.org/r/20221020223616.7571-3-phillip@squashfs.org.uk
Fixes: 8fc78b6fe2 ("squashfs: implement readahead")
Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Tested-by: Bagas Sanjaya <bagasdotme@gmail.com>
Reported-by: Marc Miltenberger <marcmiltenberger@gmail.com>
Cc: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Cc: Hsin-Yi Wang <hsinyi@chromium.org>
Cc: Mirsad Goran Todorovac <mirsad.todorovac@alu.unizg.hr>
Cc: Slade Watkins <srw@sladewatkins.net>
Cc: Thorsten Leemhuis <regressions@leemhuis.info>
Cc: <stable@vger.kernel.org>

Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/squashfs/file.c

@@ -559,6 +559,12 @@ static void squashfs_readahead(struct readahead_control *ractl)
 		unsigned int expected;
 		struct page *last_page;
 
+		expected = start >> msblk->block_log == file_end ?
+			   (i_size_read(inode) & (msblk->block_size - 1)) :
+			    msblk->block_size;
+
+		max_pages = (expected + PAGE_SIZE - 1) >> PAGE_SHIFT;
+
 		nr_pages = __readahead_batch(ractl, pages, max_pages);
 		if (!nr_pages)
 			break;
@@ -567,13 +573,10 @@ static void squashfs_readahead(struct readahead_control *ractl)
 			goto skip_pages;
 
 		index = pages[0]->index >> shift;
+
 		if ((pages[nr_pages - 1]->index >> shift) != index)
 			goto skip_pages;
 
-		expected = index == file_end ?
-			   (i_size_read(inode) & (msblk->block_size - 1)) :
-			    msblk->block_size;
-
 		if (index == file_end && squashfs_i(inode)->fragment_block !=
 						SQUASHFS_INVALID_BLK) {
 			res = squashfs_readahead_fragment(pages, nr_pages,