ext4: Fix null bh pointer dereference in mballoc

Repoted by Adrian Bunk <bunk@kernel.org>:

The Coverity checker spotted the following NULL dereference:

static int ext4_mb_mark_diskspace_used
{
	...
	if (!bitmap_bh)
		goto out_err;
	...
out_err:
	sb->s_dirt = 1;
	put_bh(bitmap_bh);
	...

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>