examples/multi_process: fix build on Ubuntu 20.04

examples/multi_process: fix build on Ubuntu 20.04

Two warnings are reported by gcc 9.3.0 on Ubuntu 20.04.

When producing a printable mac address the buffer was appropriately sized
for holding the mac address exactly, but the actual snprintf included a
'\n' character at the end, which means that the snprintf technically is
getting truncated i.e. the \n would not be added due to lack of space.
This gets flagged as a problem by modern versions of gcc, e.g. on Ubuntu
20.04.

main.c:77:37: warning: ‘__builtin___snprintf_chk’ output truncated
  before the last format character [-Wformat-truncation=]
   77 |     "%02x:%02x:%02x:%02x:%02x:%02x\n",
      |                                     ^

Since the \n is getting stripped anyway, we can fix the issue by just
removing it. In the process we can switch to using the standard ethernet
address formatting function from rte_ether.h.

The other warning is about possible string truncation when getting the
RX queue name:

In file included from init.c:36:
init.c: In function ‘init’:
../shared/common.h:38:28: warning: ‘%u’ directive output may be truncated
  writing between 1 and 10 bytes into a region of size 8
  [-Wformat-truncation=]
   38 | #define MP_CLIENT_RXQ_NAME "MProc_Client_%u_RX"
      |                            ^~~~~~~~~~~~~~~~~~~~
../shared/common.h:52:35: note: in expansion of macro ‘MP_CLIENT_RXQ_NAME’
   52 |  snprintf(buffer, sizeof(buffer), MP_CLIENT_RXQ_NAME, id);
      |                                   ^~~~~~~~~~~~~~~~~~

This is a false positive, as the value of the "id" is limited to 255,
being stored in the app as a uint8_t value, removing the possibility of
the %u being replaced by anything other then 3 characters max (rather than
up to 10 as thought by the compiler). Therefore, the warning can be easily
removed by changing the type of the "id" parameter to the local function
from "unsigned" to "uint8_t" also, ensuring the compiler is aware of the
range limit.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Radu Nicolau <radu.nicolau@intel.com>