ocfs2: Avoid unnecessary block mapping when refreshing quota info

The position of global quota file info does not change. So we do not have
to do logical -> physical block translation every time we reread it from
disk. Thus we can also avoid taking ip_alloc_sem.

Acked-by: Joel Becker <Joel.Becker@oracle.com>
Signed-off-by: Jan Kara <jack@suse.cz>