readahead: record mmap read-around states in file_ra_state

Mmap read-around now shares the same code style and data structure with
readahead code.

This also removes do_page_cache_readahead().  Its last user, mmap
read-around, has been changed to call ra_submit().

The no-readahead-if-congested logic is dumped by the way.  Users will be
pretty sensitive about the slow loading of executables.  So it's
unfavorable to disabled mmap read-around on a congested queue.

[akpm@linux-foundation.org: coding-style fixes]
Cc: Nick Piggin <npiggin@suse.de>
Signed-off-by: Fengguang Wu <wfg@mail.ustc.edu.cn>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/filemap.c

@@ -1488,13 +1488,15 @@ static void do_sync_mmap_readahead(struct vm_area_struct *vma,
 	if (ra->mmap_miss > MMAP_LOTSAMISS)
 		return;
 
+	/*
+	 * mmap read-around
+	 */
 	ra_pages = max_sane_readahead(ra->ra_pages);
 	if (ra_pages) {
-		pgoff_t start = 0;
-
-		if (offset > ra_pages / 2)
-			start = offset - ra_pages / 2;
-		do_page_cache_readahead(mapping, file, start, ra_pages);
+		ra->start = max_t(long, 0, offset - ra_pages/2);
+		ra->size = ra_pages;
+		ra->async_size = 0;
+		ra_submit(ra, mapping, file);
 	}
 }