update(driver/bpf): improve btf compatibility for 4.x kernels

Signed-off-by: Lorenzo Fontana <lo@linux.com>

Author: fntlnz

driver/bpf/filler_helpers.h

@@ -739,6 +739,8 @@ static __always_inline int __bpf_val_to_ring(struct filler_data *data,
 	if (data->state->tail_ctx.curoff > SCRATCH_SIZE_HALF)
 		return PPM_FAILURE_BUFFER_FULL;
 
+	fix_var_compat(data->state->tail_ctx.curoff);
+
 	switch (type) {
 	case PT_CHARBUF:
 	case PT_FSPATH:

driver/bpf/fillers.h

@@ -54,9 +54,11 @@ struct timeval {
 
 #else
 #include "vmlinux.h"
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 6, 0)
 #define compat_timespec __kernel_old_timespec
 #define timespec __kernel_old_timespec
 #define timeval __kernel_old_timeval
+#endif
 
 #endif // __SYSDIG_BTF_BUILD__