jbd2: don't wake kjournald unnecessarily

Don't send an extra wakeup to kjournald in the case where we
already have the proper target in j_commit_request, i.e. that
transaction has already been requested for commit.

commit deeeaf13

 "jbd2: fix fsync() tid wraparound bug" changed
the logic leading to a wakeup, but it caused some extra wakeups
which were found to lead to a measurable performance regression.

Signed-off-by: Eric Sandeen <sandeen@redhat.com>
[tytso@mit.edu: reworked check to make it clearer]
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>