bpf: Add show_fdinfo for uprobe_multi

bpf: Add show_fdinfo for uprobe_multi

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

link_type:	uprobe_multi
link_id:	9
prog_tag:	e729f789e34a8eca
prog_id:	39
uprobe_cnt:	3
pid:	0
path:	/home/dylane/bpf/tools/testing/selftests/bpf/test_progs
cookie	 offset	 ref_ctr_offset
3	 0xa69f13	 0x0
1	 0xa69f1e	 0x0
2	 0xa69f29	 0x0

Signed-off-by: Tao Chen <chen.dylane@linux.dev>
Link: https://lore.kernel.org/r/20250702153958.639852-2-chen.dylane@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>