ocfs2: Add a missing journal credit in ocfs2_link_credits() -v2

With indexed_dir enabled, ocfs2 maintains a list of dirblocks having
space.

The credit calculation in ocfs2_link_credits() did not correctly account
for adding an entry that exactly fills a dirblock that triggers removing
that dirblock by changing the pointer in the previous block in the list.
The credit calculation did not account for that previous block.

To expose, do:

mkfs.ocfs2 -b 512 -M local /dev/sdX
mount /dev/sdX /ocfs2
mkdir /ocfs2/linkdir
touch /ocfs2/linkdir/file1
for i in `seq 1 29` ; do link /ocfs2/linkdir/file1
/ocfs2/linkdir/linklinklinklinklinklink$i; done
rm -f /ocfs2/linkdir/linklinklinklinklinklink10
sleep 8
link /ocfs2/linkdir/file1
/ocfs2/linkdir/linklinklinklinklinklinkaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

Note:
The link names have been crafted for a 512 byte blocksize. Reproducing
with a larger blocksize will require longer (or more) links. The sleep
is important. We want jbd2 to commit the transaction so that the missing
block does not piggy back on account of the previous transaction.

Signed-off-by: XiaoweiHu <xiaowei.hu at oracle.com>
Reviewed-by: WengangWang <wen.gang.wang at oracle.com>
Reviewed-by: Sunil.Mushran <sunil.mushran at oracle.com>
Signed-off-by: Joel Becker <jlbec@evilplan.org>