ocfs2: fstrim: Fix start offset of first cluster group during fstrim

ocfs2: fstrim: Fix start offset of first cluster group during fstrim

The first cluster group descriptor is not stored at the start of the
group but at an offset from the start.  We need to take this into
account while doing fstrim on the first cluster group.  Otherwise we
will wrongly start fstrim a few blocks after the desired start block and
the range can cross over into the next cluster group and zero out the
group descriptor there.  This can cause filesytem corruption that cannot
be fixed by fsck.

Link: http://lkml.kernel.org/r/1507835579-7308-1-git-send-email-ashish.samant@oracle.com
Signed-off-by: Ashish Samant <ashish.samant@oracle.com>
Reviewed-by: Junxiao Bi <junxiao.bi@oracle.com>
Reviewed-by: Joseph Qi <jiangqi903@gmail.com>
Cc: Mark Fasheh <mfasheh@versity.com>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>