tools: bpftool: attempt to mount tracefs if required for tracelog cmd

As a follow-up to commit 30da46b5 ("tools: bpftool: add a command to
dump the trace pipe"), attempt to mount the tracefs virtual file system
if it is not detected on the system before trying to dump content of the
tracing pipe on an invocation of "bpftool prog tracelog".

Usually, tracefs in automatically mounted by debugfs when the user tries
to access it (e.g. "ls /sys/kernel/debug/tracing" mounts the tracefs).
So if we failed to find it, it is probably that debugfs is not here
either. Therefore, we just attempt a single mount, at a location that
does not involve debugfs: /sys/kernel/tracing.

Suggested-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>