mbuf: get the name of offload flags

mbuf: get the name of offload flags

In test-pmd (rxonly.c), the code is able to dump the list of ol_flags.
The issue is that the list of flags in the application has to be
synchronized with the flags defined in rte_mbuf.h.

This patch introduces 2 new functions rte_get_rx_ol_flag_name()
and rte_get_tx_ol_flag_name() that returns the name of a flag from
its mask. It also fixes rxonly.c to use this new functions and to
display the proper flags.

Signed-off-by: Olivier Matz <olivier.matz@6wind.com>