jbd: fix missing kernel-doc

Fix jbd header file kernel-doc notation:

Warning(linux-2.6.28-git13//include/linux/jbd.h:823): No description found for parameter 'j_average_commit_time'
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/jbd.h

@@ -614,6 +614,8 @@ struct transaction_s
  * @j_wbufsize: maximum number of buffer_heads allowed in j_wbuf, the
  *	number that will fit in j_blocksize
  * @j_last_sync_writer: most recent pid which did a synchronous write
+ * @j_average_commit_time: the average amount of time in nanoseconds it
+ *	takes to commit a transaction to the disk.
  * @j_private: An opaque pointer to fs-private information.
  */