timer: optimize apply_slack()

__fls(mask) is equivalent to find_last_bit(&mask, BITS_PER_LONG), but cheaper.
find_last_bit was showing up high on the list when I was profiling for stalls
on icache misses on a system with very small cache size (MIPS).

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
ARM: mutex: use generic atomic_dec-based implementation for ARMv6+

Commit a76d7bd96d65 ("ARM: 7467/1: mutex: use generic xchg-based
implementation for ARMv6+") removed the barrier-less, ARM-specific
mutex implementation in favour of the generic xchg-based code.

Since then, a bug was uncovered in the xchg code when running on SMP
platforms, due to interactions between the locking paths and the
MUTEX_SPIN_ON_OWNER code. This was fixed in 0bce9c46bf3b ("mutex: place
lock in contended state after fastpath_lock failure"), however, the
atomic_dec-based mutex algorithm is now marginally more efficient for
ARM (~0.5% improvement in hackbench scores on dual A15).

This patch moves ARMv6+ platforms to the atomic_dec-based mutex code.

Change-Id: I6661faeadc634a55161726ceaa75b58c3c9ff6e7
Cc: Nicolas Pitre <nico@fluxnic.net>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Nicolas Pitre <nico@linaro.org>