xfs: clear kernel only flags in XFS_IOC_ATTRMULTI_BY_HANDLE

commit 953aa9d1 upstream.

Don't allow passing arbitrary flags as they change behavior including
memory allocation that the call stack is not prepared for.

Fixes: ddbca70c

 ("xfs: allocate xattr buffer on demand")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>