bpf: Use dedicated bpf_trace_printk event instead of trace_printk()

The bpf helper bpf_trace_printk() uses trace_printk() under the hood.
This leads to an alarming warning message originating from trace
buffer allocation which occurs the first time a program using
bpf_trace_printk() is loaded.

We can instead create a trace event for bpf_trace_printk() and enable
it in-kernel when/if we encounter a program using the
bpf_trace_printk() helper.  With this approach, trace_printk()
is not used directly and no warning message appears.

This work was started by Steven (see Link) and finished by Alan; added
Steven's Signed-off-by with his permission.

Signed-off-by: Steven Rostedt (VMware) <[email protected]>
Signed-off-by: Alan Maguire <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Acked-by: Andrii Nakryiko <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Link: https://lore.kernel.org/bpf/[email protected]

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