ftrace: Don't allocate ftrace module map if ftrace is disabled

If ftrace is disabled, it is meaningless to allocate a module map.
Add a check in allocate_ftrace_mod_map() to not allocate if ftrace is
disabled.

Link: https://lore.kernel.org/20250529111955.2349189-3-yebin@huaweicloud.com


Signed-off-by: Ye Bin <yebin10@huawei.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>