tracing: Fix a NULL vs IS_ERR() bug in event_subsystem_dir()

tracing: Fix a NULL vs IS_ERR() bug in event_subsystem_dir()

commit 5264a2f4bb3baf712e19f1f053caaa8d7d3afa2e upstream.

The eventfs_create_dir() function returns error pointers, it never returns
NULL.  Update the check to reflect that.

Link: https://lore.kernel.org/linux-trace-kernel/ff641474-84e2-46a7-9d7a-62b251a1050c@moroto.mountain
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Fixes: 5790b1fb3d67 ("eventfs: Remove eventfs_file and just use eventfs_inode")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>