tracing: Modify soft-mode only if there's no other referrer

tracing: Modify soft-mode only if there's no other referrer

Modify soft-mode flag only if no other soft-mode referrer
(currently only the ftrace triggers) by using a reference
counter in each ftrace_event_file.

Without this fix, adding and removing several different
enable/disable_event triggers on the same event clear
soft-mode bit from the ftrace_event_file. This also
happens with a typo of glob on setting triggers.

e.g.

 # echo vfs_symlink:enable_event:net:netif_rx > set_ftrace_filter
 # cat events/net/netif_rx/enable
 0*
 # echo typo_func:enable_event:net:netif_rx > set_ftrace_filter
 # cat events/net/netif_rx/enable
 0
 # cat set_ftrace_filter
 #### all functions enabled ####
 vfs_symlink:enable_event:net:netif_rx:unlimited

As above, we still have a trigger, but soft-mode is gone.

Link: http://lkml.kernel.org/r/20130509054429.30398.7464.stgit@mhiramat-M0-7522
Cc: Srikar Dronamraju <[email protected]>
Cc: Oleg Nesterov <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: David Sharp <[email protected]>
Cc: Hiraku Toyooka <[email protected]>
Cc: Tom Zanussi <[email protected]>
Signed-off-by: Masami Hiramatsu <[email protected]>
Signed-off-by: Steven Rostedt <[email protected]>