samples: bpf: Fix a couple of warnings

cookie_uid_helper_example.c: In function ‘main’:
cookie_uid_helper_example.c:178:69: warning: ‘ -j ACCEPT’ directive
	writing 10 bytes into a region of size between 8 and 58
	[-Wformat-overflow=]
  178 |  sprintf(rules, "iptables -A OUTPUT -m bpf --object-pinned %s -j ACCEPT",
      |								       ^~~~~~~~~~
/home/kkd/src/linux/samples/bpf/cookie_uid_helper_example.c:178:9: note:
	‘sprintf’ output between 53 and 103 bytes into a destination of size 100
  178 |  sprintf(rules, "iptables -A OUTPUT -m bpf --object-pinned %s -j ACCEPT",
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  179 |         file);
      |         ~~~~~

Fix by using snprintf and a sufficiently sized buffer.

tracex4_user.c:35:15: warning: ‘write’ reading 12 bytes from a region of
	size 11 [-Wstringop-overread]
   35 |         key = write(1, "\e[1;1H\e[2J", 12); /* clear screen */
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~

Use size as 11.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210821002010.845777-2-memxor@gmail.com