cpumask_first() is a more effective analogue of 'next' version if n == -1
(which means start == 0). This patch replaces 'next' with 'first' where
things look trivial.

There's no cpumask_first_zero() function, so create it.
Signed-off-by: NYury Norov <yury.norov@gmail.com>
Tested-by: NWolfram Sang <wsa+renesas@sang-engineering.com>