examples/l2fwd-crypto: fix string overflow

examples/l2fwd-crypto: fix string overflow

When parsing crypto device type, the string was being copied
with strcpy(), which could overflow the destination buffer
(which is 32 byte long), so snprintf() should be used instead.

This fixes coverity issue 124575:
/examples/l2fwd-crypto/main.c: 1005 in l2fwd_crypto_parse_args_long_options()
>>>     CID 124575:    (STRING_OVERFLOW)
>>>     You might overrun the 32 byte fixed-size string
"options->string_auth_algo" by copying "optarg" without checking the length.
1005    strcpy(options->string_auth_algo, optarg);

Fixes: 49f79e86480d ("examples/l2fwd-crypto: add missing string initialization")

Signed-off-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>
Acked-by: Declan Doherty <declan.doherty@intel.com>