mbuf: fix segments number type increase

Update types of variables to correspond to nb_segs type change from
uint8_t to uint16_t.

Fixes: 97cb466d65c9 ("mbuf: use 2 bytes for port and nb segments")
Cc: stable@dpdk.org
Signed-off-by: Ilya V. Matveychikov <matvejchikov@gmail.com>
Acked-by: Olivier Matz <olivier.matz@6wind.com>