locking: Remove deprecated smp_mb__after_clear_bit()

Replace deprecated smp_mb__after_clear_bit() with smp_mb__after_atomic()
to fix following warning:
----------
arch/arm/kernel/smp.c: In function ‘smp_send_all_cpu_backtrace’:
arch/arm/kernel/smp.c:588:2: warning: ‘smp_mb__after_clear_bit’ is deprecated (declared at include/linux/bitops.h:51) [-Wdeprecated-declarations]
----------

This fix is a followup of upstream commit:
febdbfe8 (arch: Prepare for smp_mb__{before,after}_atomic())

Also with this fix in place we no longer need to support deprecated
smp_mb__() barriers and we can revert commit:
a4616e0e (Revert "locking: Remove deprecated smp_mb__() barriers")

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>