Btrfs: add rw argument to merge_bio_hook()

We'll want to merge writes so they can fill a full RAID[56] stripe, but
not necessarily reads.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>
diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 67ed24a..1b98c4c 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1566,7 +1566,7 @@
  * extent_io.c merge_bio_hook, this must check the chunk tree to make sure
  * we don't create bios that span stripes or chunks
  */
-int btrfs_merge_bio_hook(struct page *page, unsigned long offset,
+int btrfs_merge_bio_hook(int rw, struct page *page, unsigned long offset,
 			 size_t size, struct bio *bio,
 			 unsigned long bio_flags)
 {
@@ -1581,7 +1581,7 @@
 
 	length = bio->bi_size;
 	map_length = length;
-	ret = btrfs_map_block(root->fs_info, READ, logical,
+	ret = btrfs_map_block(root->fs_info, rw, logical,
 			      &map_length, NULL, 0);
 	/* Will always return 0 with map_multi == NULL */
 	BUG_ON(ret < 0);