Btrfs: Create a work queue for bio writes

This allows checksumming to happen in parallel among many cpus, and
keeps us from bogging down pdflush with the checksumming code.

Signed-off-by: Chris Mason <chris.mason@oracle.com>
diff --git a/fs/btrfs/extent_io.h b/fs/btrfs/extent_io.h
index b47859c..232300d 100644
--- a/fs/btrfs/extent_io.h
+++ b/fs/btrfs/extent_io.h
@@ -24,11 +24,12 @@
 
 struct extent_state;
 
+typedef	int (extent_submit_bio_hook_t)(struct inode *inode, int rw,
+				       struct bio *bio, int mirror_num);
 struct extent_io_ops {
 	int (*fill_delalloc)(struct inode *inode, u64 start, u64 end);
 	int (*writepage_io_hook)(struct page *page, u64 start, u64 end);
-	int (*submit_bio_hook)(struct inode *inode, int rw, struct bio *bio,
-			       int mirror_num);
+	extent_submit_bio_hook_t *submit_bio_hook;
 	int (*merge_bio_hook)(struct page *page, unsigned long offset,
 			      size_t size, struct bio *bio);
 	int (*readpage_io_hook)(struct page *page, u64 start, u64 end);