LU-1131 osd-ldiskfs: better journal credit tracking

LU-1131 osd-ldiskfs: better journal credit tracking

When running with a small MDT device during testing, it is possible to
overflow the reserved credit maximum for the journal.  Improve the
ldiskfs debugging for transaction credits so that it is possible to
better understand where the reserved credits are being used.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Iea90b771a5e19190cc95cbf8f2f725bede500c1e
Reviewed-on: http://review.whamcloud.com/4282
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Mike Pershin <tappro@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>