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>

fs/xfs/xfs_itable.c

@@ -272,6 +272,7 @@ xfs_bulkstat_to_bstat(
 	struct xfs_bstat		*bs1,
 	const struct xfs_bulkstat	*bstat)
 {
+	/* memset is needed here because of padding holes in the structure. */
 	memset(bs1, 0, sizeof(struct xfs_bstat));
 	bs1->bs_ino = bstat->bs_ino;
 	bs1->bs_mode = bstat->bs_mode;
@@ -388,6 +389,8 @@ xfs_inumbers_to_inogrp(
 	struct xfs_inogrp		*ig1,
 	const struct xfs_inumbers	*ig)
 {
+	/* memset is needed here because of padding holes in the structure. */
+	memset(ig1, 0, sizeof(struct xfs_inogrp));
 	ig1->xi_startino = ig->xi_startino;
 	ig1->xi_alloccount = ig->xi_alloccount;
 	ig1->xi_allocmask = ig->xi_allocmask;