jbd2: Fix buffer head leak when writing the commit block

jbd2: Fix buffer head leak when writing the commit block

Also make sure the buffer heads are marked clean before submitting bh
for writing.  The previous code was marking the buffer head dirty,
which would have forced an unneeded write (and seek) to the journal
for no good reason.

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