tracing/event: Cleanup the useless dentry variable

Cleanup the useless dentry variable while creating a kernel
event set of files. trace_create_file() warns if it fails to
create the file anyway, and we don't store the dentry anywhere.

v2: Fix a small conflict in kernel/trace/trace_events.c

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Li Zefan <lizf@cn.fujitsu.com>