lib/cpumask: deprecate nr_cpumask_bits

Cpumask code is written in assumption that when CONFIG_CPUMASK_OFFSTACK
is enabled, all cpumasks have boot-time defined size, otherwise the size
is always NR_CPUS.

The latter is wrong because the number of possible cpus is always
calculated on boot, and it may be less than NR_CPUS.

On my 4-cpu arm64 VM the nr_cpu_ids is 4, as expected, and nr_cpumask_bits
is 256, which corresponds to NR_CPUS. This not only leads to useless
traversing of cpumask bits greater than 4, this also makes some cpumask
routines fail.

For example, cpumask_full(0b1111000..000) would erroneously return false
in the example above because tail bits in the mask are all unset.

This patch deprecates nr_cpumask_bits and wires it to nr_cpu_ids
unconditionally, so that cpumask routines will not waste time traversing
unused part of cpu masks. It also fixes cpumask_full() and similar
routines.

As a side effect, because now a length of cpumasks is defined at run-time
even if CPUMASK_OFFSTACK is disabled, compiler can't optimize corresponding
functions.

It increases kernel size by ~2.5KB if OFFSTACK is off. This is addressed in
the following patch.

Signed-off-by: Yury Norov <yury.norov@gmail.com>