selftests: bpf: correct perror strings

perror(str) is basically equivalent to
print("%s: %s\n", str, strerror(errno)).
New line or colon at the end of str is
a mistake/breaks formatting.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>