tracing/fprobe: Fix to unregister ftrace_ops if it is empty on module unloading

tracing/fprobe: Fix to unregister ftrace_ops if it is empty on module unloading

Fix fprobe to unregister ftrace_ops if corresponding type of fprobe
does not exist on the fprobe_ip_table and it is expected to be empty
when unloading modules.

Since ftrace thinks that the empty hash means everything to be traced,
if we set fprobes only on the unloaded module, all functions are traced
unexpectedly after unloading module.
e.g.

 # modprobe xt_LOG.ko
 # echo 'f:test log_tg*' > dynamic_events
 # echo 1 > events/fprobes/test/enable
 # cat enabled_functions
log_tg [xt_LOG] (1)             tramp: 0xffffffffa0004000 (fprobe_ftrace_entry+0x0/0x490) ->fprobe_ftrace_entry+0x0/0x490
log_tg_check [xt_LOG] (1)               tramp: 0xffffffffa0004000 (fprobe_ftrace_entry+0x0/0x490) ->fprobe_ftrace_entry+0x0/0x490
log_tg_destroy [xt_LOG] (1)             tramp: 0xffffffffa0004000 (fprobe_ftrace_entry+0x0/0x490) ->fprobe_ftrace_entry+0x0/0x490
 # rmmod xt_LOG
 # wc -l enabled_functions
34085 enabled_functions

Link: https://lore.kernel.org/all/177669368776.132053.10042301916765771279.stgit@mhiramat.tok.corp.google.com/
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>