bpf: Introduce might_sleep field in bpf_func_proto

Introduce bpf_func_proto->might_sleep to indicate a particular helper
might sleep. This will make later check whether a helper might be
sleepable or not easier.

Acked-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/r/20221124053211.2373553-1-yhs@fb.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>