readahead: increase interleaved readahead size

Make sure interleaved readahead size is larger than request size.  This
also makes the readahead window grow up more quickly.
Reported-by: NXu Chenfeng <xcf@ustc.edu.cn>
Signed-off-by: NWu Fengguang <fengguang.wu@intel.com>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/readahead.c

@@ -403,6 +403,7 @@ ondemand_readahead(struct address_space *mapping,
 
 		ra->start = start;
 		ra->size = start - offset;	/* old async_size */
+		ra->size += req_size;
 		ra->size = get_next_ra_size(ra, max);
 		ra->async_size = ra->size;
 		goto readit;