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.

idea is taken here -
https://github.com/franciscofranco/flo/commit/ed2e5356068e234f407f4b3e522a48cad425d593