pktgen: make /proc/net/pktgen/pgctrl report fail on invalid input

Giving /proc/net/pktgen/pgctrl an invalid command just returns shell
success and prints a warning in dmesg.  This is not very useful for
shell scripting, as it can only detect the error by parsing dmesg.

Instead return -EINVAL when the command is unknown, as this provides
userspace shell scripting a way of detecting this.

Also bump version tag to 2.75, because (1) reading /proc/net/pktgen/pgctrl
output this version number which would allow to detect this small
semantic change, and (2) because the pktgen version tag have not been
updated since 2010.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>