sys_io_submit: simplify iocb_cmd_lookup() helper.

sys_io_submit: simplify iocb_cmd_lookup() helper.

This helper returns two values (a string and an enum).
The caller prints the string. It's simpler to just print
the string in the caller itself. This eliminates
"return by reference" and more importantly, an intermediate
static string buffer for the string result.

Since function of the helper is different now,
it is renamed to tprint_lio_opcode().

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>