tools: bpftool: remove new lines from errors

It's a little bit unusual for kernel style, but we add the new line
character to error strings inside the p_err() function.  We do this
because new lines at the end of error strings will break JSON output.

Fix a few p_err("..\n") which snuck in recently.

Fixes: 5ccda64d ("bpftool: implement cgroup bpf operations")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>