bpf: Add multi tracing attach types

Adding new program attach types multi tracing attachment:
  BPF_TRACE_FENTRY_MULTI
  BPF_TRACE_FEXIT_MULTI

and their base support in verifier code.

Programs with such attach type will use specific link attachment
interface coming in following changes.

This was suggested by Andrii some (long) time ago and turned out
to be easier than having special program flag for that.

Bpf programs with such types have 'bpf_multi_func' function set as
their attach_btf_id and keep module reference when it's specified
by attach_prog_fd.

They are also accepted as sleepable programs during verification,
and the real validation for specific BTF_IDs/functions will happen
during the multi link attachment in following changes.

Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20260606123955.345967-11-jolsa@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/uapi/linux/bpf.h

@@ -1156,6 +1156,8 @@ enum bpf_attach_type {
 	BPF_TRACE_KPROBE_SESSION,
 	BPF_TRACE_UPROBE_SESSION,
 	BPF_TRACE_FSESSION,
+	BPF_TRACE_FENTRY_MULTI,
+	BPF_TRACE_FEXIT_MULTI,
 	__MAX_BPF_ATTACH_TYPE
 };