ext4: Fixes len calculation in mpage_journal_page_buffers

Truncate operation can race with writeback, in which inode->i_size can get
truncated and therefore size - folio_pos() can be negative. This fixes the
len calculation. However this path doesn't get easily triggered even
with data journaling.

Cc: stable@kernel.org # v6.5
Fixes: 80be8c5c ("Fixes: ext4: Make mpage_journal_page_buffers use folio")
Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/cff4953b5c9306aba71e944ab176a5d396b9a1b7.1709182250.git.ritesh.list@gmail.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>