bpf: Revert "bpf: remove unnecessary rcu_read_{lock,unlock}() in multi-uprobe attach logic"

This reverts commit 4a8f635.

Althought get_pid_task() internally already calls rcu_read_lock() and
rcu_read_unlock(), the find_vpid() was not.

The documentation for find_vpid() clearly states:
"Must be called with the tasklist_lock or rcu_read_lock() held."

Add proper rcu_read_lock/unlock() to protect the find_vpid().

Fixes: 4a8f635 ("bpf: remove unnecessary rcu_read_{lock,unlock}() in multi-uprobe attach logic")
Reported-by: Xuewen Yan <xuewen.yan@unisoc.com>
Signed-off-by: Di Shen <di.shen@unisoc.com>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20250520054943.5002-1-xuewen.yan@unisoc.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Di Shen

kernel/trace/bpf_trace.c

@@ -3318,7 +3318,9 @@ int bpf_uprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *pr
 	}
 
 	if (pid) {
+		rcu_read_lock();
 		task = get_pid_task(find_vpid(pid), PIDTYPE_TGID);
+		rcu_read_unlock();
 		if (!task) {
 			err = -ESRCH;
 			goto error_path_put;