bpf: Add show_fdinfo for kprobe_multi

bpf: Add show_fdinfo for kprobe_multi

Show kprobe_multi link info with fdinfo, the info as follows:

link_type:	kprobe_multi
link_id:	1
prog_tag:	a69740b9746f7da8
prog_id:	21
kprobe_cnt:	8
missed:	0
cookie	 func
1	 bpf_fentry_test1+0x0/0x20
7	 bpf_fentry_test2+0x0/0x20
2	 bpf_fentry_test3+0x0/0x20
3	 bpf_fentry_test4+0x0/0x20
4	 bpf_fentry_test5+0x0/0x20
5	 bpf_fentry_test6+0x0/0x20
6	 bpf_fentry_test7+0x0/0x20
8	 bpf_fentry_test8+0x0/0x10

Signed-off-by: Tao Chen <chen.dylane@linux.dev>
Link: https://lore.kernel.org/r/20250702153958.639852-3-chen.dylane@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>