gfs2: setattr_chown: Add missing initialization

[ Upstream commit 2d8d7990619878a848b1d916c2f936d3012ee17d ]

Add a missing initialization of variable ap in setattr_chown().
Without, chown() may be able to bypass quotas.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>