GFS2: Add readahead to sequential directory traversal

This patch adds read-ahead capability to GFS2's
directory hash table management.  It greatly improves
performance for some directory operations.  For example:
In one of my file systems that has 1000 directories, each
of which has 1000 files, time to execute a recursive
ls (time ls -fR /mnt/gfs2 > /dev/null) was reduced
from 2m2.814s on a stock kernel to 0m45.938s.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>

fs/gfs2/file.c

@@ -105,7 +105,7 @@ static int gfs2_readdir(struct file *file, void *dirent, filldir_t filldir)
 		return error;
 	}
 
-	error = gfs2_dir_read(dir, &offset, dirent, filldir);
+	error = gfs2_dir_read(dir, &offset, dirent, filldir, &file->f_ra);
 
 	gfs2_glock_dq_uninit(&d_gh);