tick-sched: Check tick_nohz_enabled in tick_nohz_switch_to_nohz()

tick-sched: Check tick_nohz_enabled in tick_nohz_switch_to_nohz()

Since commit d689fe222 (NOHZ: Check for nohz active instead of nohz
enabled) the tick_nohz_switch_to_nohz() function returns because it
checks for the tick_nohz_active flag. This can't be set, because the
function itself sets it.

Undo the change in tick_nohz_switch_to_nohz().

Signed-off-by: Viresh Kumar <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: <[email protected]> # 3.13+
Link: http://lkml.kernel.org/r/40939c05f2d65d781b92b20302b02243d0654224.1397537987.git.viresh.kumar@linaro.org
Signed-off-by: Thomas Gleixner <[email protected]>