xsk: Fix xsk_umem_xdp_frame_sz()

Calculating the "data_hard_end" for an XDP buffer coming from AF_XDP
zero-copy mode, the return value of xsk_umem_xdp_frame_sz() is added
to "data_hard_start".

Currently, the chunk size of the UMEM is returned by
xsk_umem_xdp_frame_sz(). This is not correct, if the fixed UMEM
headroom is non-zero. Fix this by returning the chunk_size without the
UMEM headroom.

Fixes: 2a637c5b

 ("xdp: For Intel AF_XDP drivers add XDP frame_sz")
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200520192103.355233-2-bjorn.topel@gmail.com