af_xdp: change flag passed to bpf_set_link_xdp_fd

af_xdp: change flag passed to bpf_set_link_xdp_fd

Originally XDP_FLAGS_REPLACE was passed to the function.
For kernels not defining this macro (for example 5.4) in if_link.h vpp
sets it to 0.
If kernel has this macro problem appears, replace flag requires
specifying the program to be replaced. bpf_set_link_xdp_fd wraps
around __bpf_set_link_xdp_fd_replace and passes 0 as old_fd, it leads to
an error while assessing for replacement (if 0 is passed
there is no assessing for replacement).
To address this issue no flag is passed to the function, only 0.

Type: improvement
Signed-off-by: Dastin Wilski <dastin.wilski@gmail.com>
Change-Id: I3689ce7eb8c71c699f0e589111929979c2bbe213