ARC, bpf: Fix issues reported by the static analyzers

Also updated couple of comments along the way.

One of the issues reported was indeed a bug in the code:

  memset(ctx, 0, sizeof(ctx))      // original line
  memset(ctx, 0, sizeof(*ctx))     // fixed line

That was a nice catch.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202405222314.UG5F2NHn-lkp@intel.com/
Closes: https://lore.kernel.org/oe-kbuild-all/202405232036.Xqoc3b0J-lkp@intel.com/


Signed-off-by: Shahab Vahedi <shahab@synopsys.com>
Link: https://lore.kernel.org/r/20240525035628.1026-1-list+bpf@vahedi.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>