gpu: nvgpu: initialize local variable

gpu: nvgpu: initialize local variable

Initialize character array buf in gk20a_channel_ioctl() to zero
Keeping it uninitialized can result in leaking kernel stack
info to user space since we pass this buffer to UMD

Bug 1793398

Change-Id: Iffd654dbaca3b4e3c8fd2ac270d0febd01c165b8
Signed-off-by: Deepak Nibade <dnibade@nvidia.com>
Reviewed-on: http://git-master/r/1195862
(cherry picked from commit 118809f4bd07af20df2b6c012828834695a5fccf)
Reviewed-on: http://git-master/r/1201487
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Bharat Nihalani <bnihalani@nvidia.com>
Tested-by: Bharat Nihalani <bnihalani@nvidia.com>