bpftool: fix percpu maps updating

When updating a percpu map, bpftool currently copies the provided
value only into the first per CPU copy of the specified value,
all others instances are left zeroed.

This change explicitly copies the user-provided bytes to all the
per CPU instances, keeping the sub-command syntax unchanged.

v2 -> v3:
 - drop unused argument, as per Quentin's suggestion
v1 -> v2:
 - rename the helper as per Quentin's suggestion

Fixes: 71bb428f ("tools: bpf: add bpftool")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>