ocfs2: Initialize data_ac (might be used uninitialized)

CLANG found that there is a path that has data_ac uninitialized,
this place
	2917	/* This gets us the dx_root */
	2918	ret = ocfs2_reserve_new_metadata_blocks(osb, 1, &meta_ac);
	2919	if (ret) {

	3
		Taking true branch
	2920	mlog_errno(ret);
	2921	goto out;

	4
		Control jumps to line 3168
	2922	}

Goes to the out: label without data_ac being initialized.

Ciao, Marcus

Signed-Off-By: Marcus Meissner <meissner@suse.de>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Joel Becker <jlbec@evilplan.org>