Use find_first_zero_bit to find the first cleared bit in a memory region.

This also includes the following minor changes.
- Use bitmap_zero
- Reduce unnecessary atomic bitops usage
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Stephen M. Cameron <scameron@beardog.cce.hp.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>