ocfs2: fix NULL dereference for moving extents

We can't dereference "bg" before it has been assigned.  GCC should have
warned about this but "bg" was initialized to NULL.  I've fixed that as
well.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Jie Liu <jeff.liu@oracle.com>
Cc: Mark Fasheh <mfasheh@suse.com>
Cc: Joel Becker <jlbec@evilplan.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>