selftests/ftrace: Fix event filter target_func selection

The event filter function test has been failing in our internal test
farm:

| # not ok 33 event filter function - test event filtering on functions

Running the test in verbose mode indicates that this is because the test
erroneously determines that kmem_cache_free() is the most common caller
of kmem_cache_free():

  # # + cut -d: -f3 trace
  # # + sed s/call_site=([^+]*)+0x.*/1/
  # # + sort
  # # + uniq -c
  # # + sort
  # # + tail -n 1
  # # + sed s/^[ 0-9]*//
  # # + target_func=kmem_cache_free

... and as kmem_cache_free() doesn't call itself, setting this as the
filter function for kmem_cache_free() results in no hits, and
consequently the test fails:

  # # + grep kmem_cache_free trace
  # # + grep kmem_cache_free
  # # + wc -l
  # # + hitcnt=0
  # # + grep kmem_cache_free trace
  # # + grep -v kmem_cache_free
  # # + wc -l
  # # + misscnt=0
  # # + [ 0 -eq 0 ]
  # # + exit_fail

This seems to be because the system in question has tasks with ':' in
their name (which a number of kernel worker threads have). These show up
in the trace, e.g.

  test:.sh-1299    [004] .....  2886.040608: kmem_cache_free: call_site=putname+0xa4/0xc8 ptr=000000000f4d22f4 name=names_cache

... and so when we try to extact the call_site with:

  cut -d: -f3 trace | sed 's/call_site=\([^+]*\)+0x.*/\1/'

... the 'cut' command will extrace the column containing
'kmem_cache_free' rather than the column containing 'call_site=...', and
the 'sed' command will leave this unchanged. Consequently, the test will
decide to use 'kmem_cache_free' as the filter function, resulting in the
failure seen above.

Fix this by matching the 'call_site=<func>' part specifically to extract
the function name.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Reported-by: Aishwarya TCV <aishwarya.tcv@arm.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org
Cc: linux-kselftest@vger.kernel.org
Cc: linux-trace-kernel@vger.kernel.org
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>