bpf, x86: initialize the variable "first_off" in save_args()

bpf, x86: initialize the variable "first_off" in save_args()

As Dan Carpenter reported, the variable "first_off" which is passed to
clean_stack_garbage() in save_args() can be uninitialized, which can
cause runtime warnings with KMEMsan. Therefore, init it with 0.

Fixes: 473e3150e30a ("bpf, x86: allow function arguments up to 12 for TRACING")
Cc: Hao Peng <flyingpeng@tencent.com>
Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Closes: https://lore.kernel.org/bpf/09784025-a812-493f-9829-5e26c8691e07@moroto.mountain/
Signed-off-by: Menglong Dong <imagedong@tencent.com>
Link: https://lore.kernel.org/r/20230719110330.2007949-1-imagedong@tencent.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>