ocfs2: Fix check of return value of ocfs2_start_trans() in xattr.c.

On failure, ocfs2_start_trans() returns values like ERR_PTR(-ENOMEM),
so we should check whether handle is NULL. Fix them to use IS_ERR().
Jan has made the patch for other part in ocfs2(thank Jan for it), so
this is just the fix for fs/ocfs2/xattr.c.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>