function-graph: use int instead of atomic for ftrace_graph_active

Impact: cleanup

The variable ftrace_graph_active is only modified under the
ftrace_lock mutex, thus an atomic is not necessary for modification.

Reported-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

diff --cc kernel/trace/ftrace.c
Simple merge