[XFS] fix uninitialised variable bug in dquot release

gcc on ARM warns about an using an uninitialised variable
in xfs_qm_dqrele_all_inodes(). This is a real bug, but gcc
on x86_64 is not reporting this warning so it went unnoticed.

Fix the bug by bring the inode radix tree walk code up to
date with xfs_sync_inodes_ag().

SGI-PV: 987246

Signed-off-by: Dave Chinner <david@fromorbit.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>