net/af_xdp: fix build with Linux < 5.4

net/af_xdp: fix build with Linux < 5.4

Prior to this change, two implementations of rx_syscall_handler
existed although only one was needed (for the zero copy path which
is only available from kernel 5.4 and onwards). Remove the second
definition from compat.h and move the first definition back to where
it is called in the Rx function. Doing this removes a build warning
on kernels before 5.4 which complained about the second function
being defined but not used.

Fixes: 2aa51cdd559e ("net/af_xdp: fix trigger for syscall on Tx")

Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>