cpumask: relax sanity checking constraints

The cpumask_check() was unnecessarily tight, and causes problems for the
users of cpumask_next().

We have a number of users that take the previous return value of one of
the bit scanning functions and subtract one to keep it in "range".  But
since the scanning functions end up returning up to 'small_cpumask_bits'
instead of the tighter 'nr_cpumask_bits', the range really needs to be
using that widened form.

[ This "previous-1" behavior is also the reason we have all those
  comments about /* -1 is a legal arg here. */ and separate checks for
  that being ok.  So we could have just made "small_cpumask_bits-1"
  be a similar special "don't check this" value.

  Tetsuo Handa even suggested a patch that only does that for
  cpumask_next(), since that seems to be the only actual case that
  triggers, but that all makes it even _more_ magical and special. So
  just relax the check ]

One example of this kind of pattern being the 'c_start()' function in
arch/x86/kernel/cpu/proc.c, but also duplicated in various forms on
other architectures.

Reported-by: syzbot+96cae094d90877641f32@syzkaller.appspotmail.com
Link: https://syzkaller.appspot.com/bug?extid=96cae094d90877641f32
Reported-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
Link: https://lore.kernel.org/lkml/c1f4cc16-feea-b83c-82cf-1a1f007b7eb9@I-love.SAKURA.ne.jp/
Fixes: 596ff4a09b ("cpumask: re-introduce constant-sized cpumask optimizations")
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/cpumask.h

@@ -147,7 +147,7 @@ static __always_inline void cpu_max_bits_warn(unsigned int cpu, unsigned int bit
 /* verify cpu argument to cpumask_* operators */
 static __always_inline unsigned int cpumask_check(unsigned int cpu)
 {
-	cpu_max_bits_warn(cpu, nr_cpumask_bits);
+	cpu_max_bits_warn(cpu, small_cpumask_bits);
 	return cpu;
 }