bpf: Use LINUX_KERNEL_VERSION extern variable

bpf/reuseport_kern.c

@@ -24,7 +24,6 @@
  */
 #include <linux/udp.h>
 #include <linux/bpf.h>
-#include <linux/version.h>
 
 #include <bpf/bpf_helpers.h>
 
@@ -235,22 +234,29 @@ static void InvMixColumns(state_t *state) {
   }
 }
 
+extern __u32 LINUX_KERNEL_VERSION __kconfig;
+
 /* The SubBytes Function Substitutes the values in the state matrix
    with values in an S-box. */
 static void InvSubBytes(state_t *state) {
   __u8 i, j;
+  if (LINUX_KERNEL_VERSION < KERNEL_VERSION(5, 10, 0)) {
     for (i = 0; i < 4; ++i) {
       for (j = 0; j < 4; ++j) {
-#if LINUX_VERSION_CODE < KERNEL_VERSION(5, 10, 0)
-      /* Ubuntu 20.04 LTS kernel 5.4.0 needs this workaround otherwise
-         "math between map_value pointer and register with unbounded
-         min value is not allowed".  5.10.0 is a kernel version that
-         works but it might not be a minimum version.  */
+        /* Ubuntu 20.04 LTS kernel 5.4.0 needs this workaround
+           otherwise "math between map_value pointer and register with
+           unbounded min value is not allowed".  5.10.0 is a kernel
+           version that works but it might not be the minimum
+           version.  */
         __u8 k = (*state)[j][i];
         (*state)[j][i] = k ? getSBoxInvert(k) : getSBoxInvert(0);
-#else  /* !(LINUX_VERSION_CDOE < KERNEL_VERSION(5, 10, 0)) */
+      }
+    }
+  } else {
+    for (i = 0; i < 4; ++i) {
+      for (j = 0; j < 4; ++j) {
         (*state)[j][i] = getSBoxInvert((*state)[j][i]);
-#endif /* !(LINUX_VERSION_CDOE < KERNEL_VERSION(5, 10, 0)) */
+      }
     }
   }
 }