The send() syscall on the Tx path is not concerned with busy polling
and as such its invocation should not depend on whether or not it is
configured. Fix this by distinguishing the conditions necessary for
syscalls on the Rx and Tx paths individually.
Fixes: 055a393626ed ("net/af_xdp: prefer busy polling")
Signed-off-by: Ciara Loftus <ciara.loftus@intel.com>