Fix: 4.10 hotplug adaptation backward compat

Fix: 4.10 hotplug adaptation backward compat

                 from /home/compudj/git/lttng-modules/lttng-context-perf-counters.c:23:
/home/compudj/git/lttng-modules/lttng-context-perf-counters.c: In function ‘lttng_add_perf_counter_to_ctx’:
/home/compudj/git/lttng-modules/lttng-context-perf-counters.c:353:22: error: ‘cpu’ undeclared (first use in this function)
  for_each_online_cpu(cpu) {
                      ^
./include/linux/cpumask.h:223:8: note: in definition of macro ‘for_each_cpu’
  for ((cpu) = -1;    \
        ^
/home/compudj/git/lttng-modules/lttng-context-perf-counters.c:353:2: note: in expansion of macro ‘for_each_online_cpu’
  for_each_online_cpu(cpu) {
  ^
/home/compudj/git/lttng-modules/lttng-context-perf-counters.c:353:22: note: each undeclared identifier is reported only once for each function it appears in
  for_each_online_cpu(cpu) {
                      ^
./include/linux/cpumask.h:223:8: note: in definition of macro ‘for_each_cpu’
  for ((cpu) = -1;    \
        ^
/home/compudj/git/lttng-modules/lttng-context-perf-counters.c:353:2: note: in expansion of macro ‘for_each_online_cpu’
  for_each_online_cpu(cpu) {
  ^
./include/linux/cpumask.h:224:38: warning: left-hand operand of comma expression has no effect [-Wunused-value]
   (cpu) = cpumask_next((cpu), (mask)), \
                                      ^
./include/linux/cpumask.h:717:36: note: in expansion of macro ‘for_each_cpu’
 #define for_each_online_cpu(cpu)   for_each_cpu((cpu), cpu_online_mask)
                                    ^
/home/compudj/git/lttng-modules/lttng-context-perf-counters.c:353:2: note: in expansion of macro ‘for_each_online_cpu’
  for_each_online_cpu(cpu) {
  ^
scripts/Makefile.build:289: recipe for target '/home/compudj/git/lttng-modules/lttng-context-perf-counters.o' failed
make[2]: *** [/home/compudj/git/lttng-modules/lttng-context-perf-counters.o] Error 1
make[2]: *** Waiting for unfinished jobs....

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>