xfs: fix stack contents leakage in the v1 inumber ioctls

Explicitly initialize the onstack structures to zero so we don't leak
kernel memory into userspace when converting the in-core inumbers
structure to the v1 inogrp ioctl structure.  Add a comment about why we
have to use memset to ensure that the padding holes in the structures
are set to zero.

Fixes: 5f19c7fc ("xfs: introduce v5 inode group structure")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>