tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM_LEN

tools/testing/selftests/bpf: replace open-coded 16 with TASK_COMM_LEN

As the sched:sched_switch tracepoint args are derived from the kernel,
we'd better make it same with the kernel.  So the macro TASK_COMM_LEN is
converted to type enum, then all the BPF programs can get it through
BTF.

The BPF program which wants to use TASK_COMM_LEN should include the
header vmlinux.h.  Regarding the test_stacktrace_map and
test_tracepoint, as the type defined in linux/bpf.h are also defined in
vmlinux.h, so we don't need to include linux/bpf.h again.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Yafang Shao <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Michal Miroslaw <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: David Hildenbrand <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Kees Cook <[email protected]>
Cc: Petr Mladek <[email protected]>
Cc: Alexei Starovoitov <[email protected]>
Cc: Dennis Dalessandro <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>