selftests/ftrace: Fix multiple kprobe testcase

Fix multiple kprobe event testcase to work it correctly.
There are 2 bugfixes.
- Since `wc -l FILE` returns not only line number but also
FILE filename, following "if" statement always failed.
Fix this bug by replacing it with 'cat FILE | wc -l'
- Since "while do-done loop" block with pipeline becomes a
subshell, $N local variable is not update outside of
the loop.
Fix this bug by using actual target number (256) instead
of $N.

Signed-off-by: Masami Hiramatsu <[e-mail]>
Reviewed-by: Steven Rostedt (VMware) <[e-mail]>
Signed-off-by: Shuah Khan <[e-mail]>

tools/testing/selftests/ftrace/test.d/kprobe/multiple_kprobes.tc

···
   test $N -eq 256 && break
 done
 
-L=`wc -l kprobe_events`
-if [ $L -ne $N ]; then
-  echo "The number of kprobes events ($L) is not $N"
+L=`cat kprobe_events | wc -l`
+if [ $L -ne 256 ]; then
+  echo "The number of kprobes events ($L) is not 256"
   exit_fail
 fi