mutex: Avoid label warning when !CONFIG_MUTEX_SPIN_ON_OWNER

Fengguang reported the following warning when optimistic
spinning is disabled (ie: make allnoconfig):

   kernel/mutex.c:599:1: warning: label 'done' defined but not used

Remove the 'done' label altogether.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Davidlohr Bueso <davidlohr.bueso@hp.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>