bpf: fix check of allowed specifiers in bpf_trace_printk

A format string consisting of "%p" or "%s" followed by an invalid
specifier (e.g. "%p%\n" or "%s%") could pass the check which
would make format_decode (lib/vsprintf.c) to warn.

Fixes: 9c959c86

 ("tracing: Allow BPF programs to call bpf_trace_printk()")
Reported-by:  <syzbot+1ec5c5ec949c4adaa0c4@syzkaller.appspotmail.com>
Signed-off-by: Martynas Pumputis <m@lambda.lt>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>