ext4: Update comment in mpage_prepare_extent_to_map()

Since filemap_write_and_wait() is now enough to get journalled data to
final location update the comment in mpage_prepare_extent_to_map().

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230329154950.19720-12-jack@suse.cz


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