[test][python] temptatively attach to blk_start_request probe.

`blk_start_request` is gone since kernel 5. This patch is in the same vein as iovisor#4124
except that we only conditionally attach to `blk_start_request` but attach also attach to
`blk_mq_start_request`.

Fixes iovisor#4206

tests/python/test_trace3.py

@@ -16,11 +16,13 @@
 
 
 class TestBlkRequest(TestCase):
- @mayFail("This fails on github actions environment, and needs to be fixed")
  def setUp(self):
  b = BPF(arg1, arg2, debug=0)
  self.latency = b.get_table("latency", c_uint, c_ulong)
- b.attach_kprobe(event="blk_start_request",
+ if BPF.get_kprobe_functions(b"blk_start_request"):
+ b.attach_kprobe(event="blk_start_request",
+ fn_name="probe_blk_start_request")
+ b.attach_kprobe(event="blk_mq_start_request",
  fn_name="probe_blk_start_request")
  b.attach_kprobe(event="blk_update_request",
  fn_name="probe_blk_update_request")