[XFS] Zero uninitialised xfs_da_args structure in xfs_dir2.c

Fixes a problem in the xfs_dir2_remove and xfs_dir2_replace paths which
intenally call directory format specific lookup funtions that assume
args->cmpresult is zeroed.

SGI-PV: 982606
SGI-Modid: xfs-linux-melb:xfs-kern:31268a

Signed-off-by: Barry Naujok <bnaujok@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>