bpfloader: Allow overriding kernel version

This allows devices to pretend that they are on a different kernel version, in order to support different bpf features when kernel backports are in place.

Test: set ro.bpf.kver_override=5.10.239 in product.prop on sm8250 device
      based on 4.19 kernel with 5.10 bpf backported, skfilter_oom_kill
      loaded: bpftool prog | grep skfilter_oom_k

Change-Id: Ic32a279c562679408728483d5a5ed79a70af217d

loader/bpfloader.rs

@@ -426,6 +426,10 @@ fn parse_release(release: &str) -> (u32, u32, u32) {
 }
 
 fn kernel_version() -> Result<u32, anyhow::Error> {
+    if let Some(kver_override) = system_properties::read("ro.bpf.kver_override")? {
+        let (major, minor, sub) = parse_release(&kver_override);
+        return Ok(kver(major, minor, sub));
+    }
     let mut buf: MaybeUninit<utsname> = MaybeUninit::zeroed();
     // SAFETY: If uname returns 0, the buf should be properly initialized.
     if unsafe { uname(buf.as_mut_ptr()) } != 0 {