Btrfs: tune btrfs unplug functions for a small number of devices

When btrfs unplugs, it tries to find the correct device to unplug
via search through the extent_map tree.  This avoids unplugging
a device that doesn't need it, but is a waste of time for filesystems
with a small number of devices.

This patch checks the total number of devices before doing the
search.
Signed-off-by: NChris Mason <chris.mason@oracle.com>

fs/btrfs/disk-io.c

@@ -1201,6 +1201,16 @@ void btrfs_unplug_io_fn(struct backing_dev_info *bdi, struct page *page)
 		return;
 
 	inode = mapping->host;
+
+	/*
+	 * don't do the expensive searching for a small number of
+	 * devices
+	 */
+	if (BTRFS_I(inode)->root->fs_info->fs_devices->open_devices <= 2) {
+		__unplug_io_fn(bdi, page);
+		return;
+	}
+
 	offset = page_offset(page);
 
 	em_tree = &BTRFS_I(inode)->extent_tree;