mbuf: remove experimental tag for external attachment
mbuf: remove experimental tag for external attachment

Remove the experimental tag of rte_pktmbuf_attach_extbuf() which was
introduced in 18.05.

Signed-off-by: Yongseok Koh <76b37c91816d73ef9cf709e3e83a68f2d162f1e2@mellanox.com>
Acked-by: Andrew Rybchenko <ac94ab2a8fe9a9a087f6e24dcbc16626b52c07e8@solarflare.com>
Acked-by: Olivier Matz <dfcc1510895413197abb4336e39bb0a3906cba71@6wind.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk