bpf: Add bpf_task_from_pid() kfunc

Callers can currently store tasks as kptrs using bpf_task_acquire(),
bpf_task_kptr_get(), and bpf_task_release(). These are useful if a
caller already has a struct task_struct *, but there may be some callers
who only have a pid, and want to look up the associated struct
task_struct * from that to e.g. find task->comm.

This patch therefore adds a new bpf_task_from_pid() kfunc which allows
BPF programs to get a struct task_struct * kptr from a pid.

Signed-off-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20221122145300.251210-2-void@manifault.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>