commit de40f033d4e84e843d6a12266e3869015ea9097c upstream.

Have create_var_ref() manage the hist trigger's var_ref list, rather
than having similar code doing it in multiple places.  This cleans up
the code and makes sure var_refs are always accounted properly.

Also, document the var_ref-related functions to make what their
purpose clearer.

Link: http://lkml.kernel.org/r/05ddae93ff514e66fc03897d6665231892939913.1545161087.git.tom.zanussi@linux.intel.comAcked-by: NNamhyung Kim <namhyung@kernel.org>
Reviewed-by: NMasami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: NTom Zanussi <tom.zanussi@linux.intel.com>
Signed-off-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NYang Yingliang <yangyingliang@huawei.com>