tracing_map_elt_alloc() returns ERR_PTRs on error, never NULL.

Fixes: 08d43a5f ('tracing: Add lock-free tracing_map')
Link: http://lkml.kernel.org/r/20160423102347.GA11136@mwandaAcked-by: NTom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>