xdp: Handle frame_sz in xdp_convert_zc_to_xdp_frame()

In commit 34cc0b33 we only handled the frame_sz in convert_to_xdp_frame().
This patch will also handle frame_sz in xdp_convert_zc_to_xdp_frame().

Fixes: 34cc0b33 ("xdp: Xdp_frame add member frame_sz and handle in convert_to_xdp_frame")
Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20200616103518.2963410-1-liuhangbin@gmail.com