bpf: bpf task work plumbing

This patch adds necessary plumbing in verifier, syscall and maps to
support handling new kfunc bpf_task_work_schedule and kernel structure
bpf_task_work. The idea is similar to how we already handle bpf_wq and
bpf_timer.
verifier changes validate calls to bpf_task_work_schedule to make sure
it is safe and expected invariants hold.
btf part is required to detect bpf_task_work structure inside map value
and store its offset, which will be used in the next patch to calculate
key and value addresses.
arraymap and hashtab changes are needed to handle freeing of the
bpf_task_work: run code needed to deinitialize it, for example cancel
task_work callback if possible.
The use of bpf_task_work and proper implementation for kfuncs are
introduced in the next patch.

Signed-off-by: Mykyta Yatsenko <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Acked-by: Eduard Zingerman <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

Author: mykyta5

include/uapi/linux/bpf.h

@@ -7436,6 +7436,10 @@ struct bpf_timer {
 	__u64 __opaque[2];
 } __attribute__((aligned(8)));
 
+struct bpf_task_work {
+	__u64 __opaque;
+} __attribute__((aligned(8)));
+
 struct bpf_wq {
 	__u64 __opaque[2];
 } __attribute__((aligned(8)));