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>
Signed-off-by: edoko <r_data@naver.com>