xfs: prevent leaking uninitialized stack memory in FSGEOMETRY_V1

xfs: prevent leaking uninitialized stack memory in FSGEOMETRY_V1

commit 3a3675b7f23f83ca8c67c9c2b6edf707fd28d1ba upstream.

The FSGEOMETRY_V1 ioctl (and its compat equivalent) calls out to
xfs_fs_geometry() with a version number of 3.  This code path does not
fill in the logsunit member of the passed xfs_fsop_geom_t, leading to
the leaking of four bytes of uninitialized stack data to potentially
unprivileged callers.

v2 switches to memset() to avoid future issues if structure members
change, on suggestion of Dave Chinner.

Signed-off-by: Dan Rosenberg <drosenberg@vsecurity.com>
Reviewed-by: Eugene Teo <eugeneteo@kernel.org>
Signed-off-by: Alex Elder <aelder@sgi.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>