Using bitshifts instead of division and multiplication should improve
performance. That requires weight and factor to be powers of two, but i think
this is something we can live with.

Thanks to Peter Zijlstra for the improved formula!
Signed-off-by: NBruno Randolf <br1@einfach.org>

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v2:	use log2.h functions
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>