xdp: remove redundant variable 'headroom'

Variable 'headroom' is being assigned but is never used hence it is
redundant and can be removed.

Cleans up clang warning:
variable ‘headroom’ set but not used [-Wunused-but-set-variable]

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>