xfs: set XFS_DA_OP_OKNOENT in xfs_attr_get

It's entirely possible for userspace to ask for an xattr which
does not exist.

Normally, there is no problem whatsoever when we ask for such
a thing, but when we look at an obfuscated metadump image
on a debug kernel with selinux, we trip over this ASSERT in
xfs_da3_path_shift():

        *result = -ENOENT;      /* we're out of our tree */
        ASSERT(args->op_flags & XFS_DA_OP_OKNOENT);

It (more or less) only shows up in the above scenario, because
xfs_metadump obfuscates attr names, but chooses names which
keep the same hash value - and xfs_da3_node_lookup_int does:

        if (((retval == -ENOENT) || (retval == -ENOATTR)) &&
            (blk->hashval == args->hashval)) {
                error = xfs_da3_path_shift(state, &state->path, 1, 1,
                                                 &retval);

IOWS, we only get down to the xfs_da3_path_shift() ASSERT
if we are looking for an xattr which doesn't exist, but we
find xattrs on disk which have the same hash, and so might be
a hash collision, so we try the path shift.  When *that*
fails to find what we're looking for, we hit the assert about
XFS_DA_OP_OKNOENT.

Simply setting XFS_DA_OP_OKNOENT in xfs_attr_get solves this
rather corner-case problem with no ill side effects.  It's
fine for an attr name lookup to fail.
Signed-off-by: NEric Sandeen <sandeen@redhat.com>
Reviewed-by: NDave Chinner <dchinner@redhat.com>
Signed-off-by: NDave Chinner <david@fromorbit.com>

fs/xfs/libxfs/xfs_attr.c

@@ -139,6 +139,8 @@ xfs_attr_get(
 
 	args.value = value;
 	args.valuelen = *valuelenp;
+	/* Entirely possible to look up a name which doesn't exist */
+	args.op_flags = XFS_DA_OP_OKNOENT;
 
 	lock_mode = xfs_ilock_attr_map_shared(ip);
 	if (!xfs_inode_hasattr(ip))