tracing: Fix code comment for ftrace_ops_get_func()

tracing: Fix code comment for ftrace_ops_get_func()

There is no function 'ftrace_ops_recurs_func' existing in the current code,
it was renamed to ftrace_ops_assist_func() in commit 58061a36f2a8
("ftrace: Have ftrace_ops_get_func() handle RCU and PER_CPU flags too").
Update the comment to the correct function name.

Link: http://lkml.kernel.org/r/1487723366-14463-1-git-send-email-chuhu@redhat.com
Signed-off-by: Chunyu Hu <chuhu@redhat.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>