sysctl: Fix underflow value setting risk in vm_table

sysctl: Fix underflow value setting risk in vm_table

Commit 3b3376f222e3 ("sysctl.c: fix underflow value setting risk in vm_table")
fixes underflow value setting risk in vm_table but misses vdso_enabled
sysctl.

vdso_enabled sysctl is initialized with .extra1 value as SYSCTL_ZERO to
avoid negative value writes but the proc_handler is proc_dointvec and
not proc_dointvec_minmax and thus do not uses .extra1 and .extra2.

The following command thus works :

`# echo -1 > /proc/sys/vm/vdso_enabled`

This patch properly sets the proc_handler to proc_dointvec_minmax.
In addition to .extra1, .extra2 is set to SYSCTL_ONE. The sysctl is
thus bounded between 0 and 1.

Fixes: 3b3376f222e3 ("sysctl.c: fix underflow value setting risk in vm_table")
Signed-off-by: Nicolas Bouchinet <nicolas.bouchinet@ssi.gouv.fr>
Reviewed-by: Jan Kara <jack@suse.cz>
Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Joel Granados <joel.granados@kernel.org>