mm/readahead.c: simplify get_next_ra_size()

It's a trivial simplification for get_next_ra_size() and clear enough for
humans to understand.

It also fixes potential overflow if ra->size(< ra_pages) is too large.

Link: http://lkml.kernel.org/r/1540707206-19649-1-git-send-email-hsiangkao@aol.com
Signed-off-by: Gao Xiang <hsiangkao@aol.com>
Reviewed-by: Fengguang Wu <fengguang.wu@intel.com>
Reviewed-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/readahead.c

@@ -270,17 +270,15 @@ static unsigned long get_init_ra_size(unsigned long size, unsigned long max)
  *  return it as the new window size.
  */
 static unsigned long get_next_ra_size(struct file_ra_state *ra,
-						unsigned long max)
+				      unsigned long max)
 {
 	unsigned long cur = ra->size;
-	unsigned long newsize;
 
 	if (cur < max / 16)
-		newsize = 4 * cur;
-	else
-		newsize = 2 * cur;
-
-	return min(newsize, max);
+		return 4 * cur;
+	if (cur <= max / 2)
+		return 2 * cur;
+	return max;
 }
 
 /*