mm, vmalloc: use well-defined find_last_bit() func

mm, vmalloc: use well-defined find_last_bit() func

Our intention in here is to find last_bit within the region to flush.
There is well-defined function, find_last_bit() for this purpose and its
performance may be slightly better than current implementation.  So change
it.

Signed-off-by: Joonsoo Kim <[email protected]>
Reviewed-by: Wanpeng Li <[email protected]>
Acked-by: Johannes Weiner <[email protected]>
Acked-by: Zhang Yanfei <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>