tracing/branch: Use pr_warn() instead of printk(KERN_WARNING)

tracing/branch: Use pr_warn() instead of printk(KERN_WARNING)

Use pr_warn() instead of printk(KERN_WARNING ...) for the branch tracer
warning messages.

Keep the message text unchanged. The change only removes the open-coded
log level from these warnings.

Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20260519081620.3874441-1-pengyu@kylinos.cn
Signed-off-by: Yu Peng <pengyu@kylinos.cn>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>