Impact: have simple_read_from_buffer conform to standards

It was brought to my attention by Andrew Morton, Theodore Tso, and H. 
Peter Anvin that a read from userspace should only return -EFAULT if
nothing was actually read.

Looking at the simple_read_from_buffer I noticed that this function does
not conform to that rule.  This patch fixes that function.

[akpm@linux-foundation.org: simplification suggested by hpa]
[hpa@zytor.com: fix count==0 handling]
Signed-off-by: Steven Rostedt <srostedt@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/libfs.c

@@ -527,14 +527,18 @@ ssize_t simple_read_from_buffer(void __user *to, size_t count, loff_t *ppos,
 				const void *from, size_t available)
 {
 	loff_t pos = *ppos;
+	size_t ret;
+
 	if (pos < 0)
 		return -EINVAL;
-	if (pos >= available)
+	if (pos >= available || !count)
 		return 0;
 	if (count > available - pos)
 		count = available - pos;
-	if (copy_to_user(to, from + pos, count))
+	ret = copy_to_user(to, from + pos, count);
+	if (ret == count)
 		return -EFAULT;
+	count -= ret;
 	*ppos = pos + count;
 	return count;
 }