[PATCH] trivial, document that div64_u64() is not precise on 32bit platforms

[Oleg Nesterov <oleg@redhat.com>]

We have a bugreport which blames div64_u64() on 32bit platforms.

However, the code obviously doesn't even try to pretend it can do
the 64bit division precisely. If there is something in the high
word of divisor, div64_u64() just shifts both arguments and throws
out the low bits.

Add a small comment to avoid the confusion.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---

 lib/div64.c |    5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

--- a/lib/div64.c
+++ b/lib/div64.c
@@ -77,7 +77,10 @@ s64 div_s64_rem(s64 dividend, s32 diviso
 EXPORT_SYMBOL(div_s64_rem);
 #endif
 
-/* 64bit divisor, dividend and result. dynamic precision */
+/*
+ * 64bit divisor, dividend and result. Dynamic precision, unless
+ * divisor fits in u32 result is not exactly correct.
+ */
 #ifndef div64_u64
 u64 div64_u64(u64 dividend, u64 divisor)
 {