ocfs2: add necessary check in case sb_getblk() fails

sb_getblk() may return an err, so add a check for bh.

[joseph.qi@huawei.com: also add a check after calling sb_getblk() in ocfs2_create_xattr_block()]
Signed-off-by: Rui Xiang <rui.xiang@huawei.com>
Reviewed-by: Jie Liu <jeff.liu@oracle.com>
Reviewed-by: Mark Fasheh <mfasheh@suse.de>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Joseph Qi <joseph.qi@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>