linux/bpf_prog_load: support custom kernel builds

for custom-built kernels, kernel.version may not
always contain the kernel version number
also support kernel versions with more than one
digits between dots

syscall/linux/syscalls.lua

@@ -787,9 +787,9 @@ if C.bpf then
  if not version then
  -- We have no better way to extract current kernel hex-string other
  -- than parsing headers, compiling a helper function or reading /proc
- local ver_str, count = S.sysctl('kernel.version'):match('%d.%d.%d'), 2
+ local ver_str, count = S.sysctl('kernel.osrelease'):match('%d+.%d+.%d+'), 2
  version = 0
- for i in ver_str:gmatch('%d') do -- Convert 'X.Y.Z' to 0xXXYYZZ
+ for i in ver_str:gmatch('%d+') do -- Convert 'X.Y.Z' to 0xXXYYZZ
  version = bit.bor(version, bit.lshift(tonumber(i), 8*count))
  count = count - 1
  end