Btrfs: only map pages if we know we need them when reading the space cache

People have been running into a warning when loading space cache because the
page is already mapped when trying to read in a bitmap.  The way we read in
entries and pages is kind of convoluted, so fix it so that io_ctl_read_entry
maps the entries if it needs to, and if it hits the end of the page it simply
unmaps the page.  That way we can unconditionally unmap the io_ctl before
reading in the bitmap and we should stop hitting these warnings.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/free-space-cache.c

@@ -537,6 +537,13 @@ static int io_ctl_read_entry(struct io_ctl *io_ctl,
 			    struct btrfs_free_space *entry, u8 *type)
 {
 	struct btrfs_free_space_entry *e;
+	int ret;
+
+	if (!io_ctl->cur) {
+		ret = io_ctl_check_crc(io_ctl, io_ctl->index);
+		if (ret)
+			return ret;
+	}
 
 	e = io_ctl->cur;
 	entry->offset = le64_to_cpu(e->offset);
@@ -550,10 +557,7 @@ static int io_ctl_read_entry(struct io_ctl *io_ctl,
 
 	io_ctl_unmap_page(io_ctl);
 
-	if (io_ctl->index >= io_ctl->num_pages)
-		return 0;
-
-	return io_ctl_check_crc(io_ctl, io_ctl->index);
+	return 0;
 }
 
 static int io_ctl_read_bitmap(struct io_ctl *io_ctl,
@@ -561,9 +565,6 @@ static int io_ctl_read_bitmap(struct io_ctl *io_ctl,
 {
 	int ret;
 
-	if (io_ctl->cur && io_ctl->cur != io_ctl->orig)
-		io_ctl_unmap_page(io_ctl);
-
 	ret = io_ctl_check_crc(io_ctl, io_ctl->index);
 	if (ret)
 		return ret;
@@ -699,6 +700,8 @@ int __load_free_space_cache(struct btrfs_root *root, struct inode *inode,
 		num_entries--;
 	}
 
+	io_ctl_unmap_page(&io_ctl);
+
 	/*
 	 * We add the bitmaps at the end of the entries in order that
 	 * the bitmap entries are added to the cache.