cpumask: Mark functions as pure

cpumask_next_and() and cpumask_any_but() are pure, and marking them as
such seems to generate different and presumably better code for
native_flush_tlb_multi().

Signed-off-by: Nadav Amit <namit@vmware.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://lore.kernel.org/r/20210220231712.2475218-8-namit@vmware.com