lisa/trace: Fix dataframe prefiltering for userspace ftrace events

It looks like the appearance of userspace generated ftrace events has
changed. What appears to be memory address has been added to the event
meta data breaking the prefiltering of trace data in
_select_userspace().

The event used contain the keyword 'tracing_mark_write:', but now has
the format: 'tracing_mark_write.<address>:'.

Example:
   rt-app-21908 [001] 4470442.464974: print:
tracing_mark_write.10e87ffbd993ed977ebcbff93a794b6d: rtapp_main:
event=end

Fix the event filtering to handle new format without breaking backwards
compatibility.

lisa/trace.py

@@ -3535,7 +3535,7 @@ def _select_userspace(self, source_event, meta_event, df):
         # That said, it's not the end of the world if we don't filter on that
         # as the meta event name is supposed to be unique anyway
         if not is_numeric_dtype(df['ip'].dtype):
-            df = df[df['ip'] == 'tracing_mark_write']
+            df = df[df['ip'].str.startswith('tracing_mark_write')]
         return (df, 'buf')
 
     def _select_trace_printk(self, source_event, meta_event, df):