ocfs2: Fix check of return value of ocfs2_start_trans()

On failure, ocfs2_start_trans() returns values like ERR_PTR(-ENOMEM).
Thus checks for !handle are wrong. Fix them to use IS_ERR().

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