Use of invalid format specifiers in bpf_trace_printk should error

[drzaeus77]

Doing something like bpf_trace_printk("%0.2f" ...) silently fails. The valid format specifiers are %d, %u, %p, and %x. The frontend should be able to easily scan for such format strings and raise an error, or at least a warning, to the user, as the kernel verifier will otherwise reject the printk and return -EINVAL.