Partially revert "kfifo: fix kfifo_alloc() and kfifo_init()"

Commit dfe2a77f ("kfifo: fix kfifo_alloc() and kfifo_init()") made
the kfifo code round the number of elements up.  That was good for
__kfifo_alloc(), but it's actually wrong for __kfifo_init().

The difference? __kfifo_alloc() will allocate the rounded-up number of
elements, but __kfifo_init() uses an allocation done by the caller.  We
can't just say "use more elements than the caller allocated", and have
to round down.

The good news? All the normal cases will be using power-of-two arrays
anyway, and most users of kfifo's don't use kfifo_init() at all, but one
of the helper macros to declare a KFIFO that enforce the proper
power-of-two behavior.  But it looks like at least ibmvscsis might be
affected.

The bad news? Will Deacon refers to an old thread and points points out
that the memory ordering in kfifo's is questionable.  See

  https://lore.kernel.org/lkml/20181211034032.32338-1-yuleixzhang@tencent.com/

for more.

Fixes: dfe2a77f ("kfifo: fix kfifo_alloc() and kfifo_init()")
Reported-by: Nlaokz <laokz@foxmail.com>
Cc: Stefani Seibold <stefani@seibold.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Greg KH <greg@kroah.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

lib/kfifo.c

@@ -68,7 +68,8 @@ int __kfifo_init(struct __kfifo *fifo, void *buffer,
 {
 	size /= esize;
 
-	size = roundup_pow_of_two(size);
+	if (!is_power_of_2(size))
+		size = rounddown_pow_of_two(size);
 
 	fifo->in = 0;
 	fifo->out = 0;