ext4: pair trace_ext4_writepages & trace_ext4_writepages_result

Pair the two trace events to make troubeshooting writepages
easier, and it should be more convinient to write a simple script
to parse the traces.

Cc: linux-ext4@vger.kernel.org
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>