UPSTREAM: tracing: Use IS_ERR() check for return value of tracing_init_dentry()

tracing_init_dentry() will soon return NULL as a valid pointer for the
top level tracing directroy. NULL can not be used as an error value.
Instead, switch to ERR_PTR() and check the return status with
IS_ERR().

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Bug: 31856701
Change-Id: Ia072ac0f4dc970a0d219b21d219730afbd3d0bf0
(cherry picked from commit 14a5ae40f0def33a422a45b2ed09198adb7bf11c)
Signed-off-by: Paul Lawrence <paullawrence@google.com>