Btrfs: convert to the new truncate sequence

->truncate() is going away, instead all of the work needs to be done in
->setattr().  So this converts us over to do this.  It's fairly straightforward,
just get rid of our .truncate inode operation and call btrfs_truncate() directly
from btrfs_setsize.  This works out better for us since truncate can technically
return ENOSPC, and before we had no way of letting anybody know.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
diff --git a/fs/btrfs/file.c b/fs/btrfs/file.c
index 24a19c2..3786eca 100644
--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -817,7 +817,7 @@
 	last_pos = ((u64)index + num_pages) << PAGE_CACHE_SHIFT;
 
 	if (start_pos > inode->i_size) {
-		err = btrfs_cont_expand(inode, start_pos);
+		err = btrfs_cont_expand(inode, i_size_read(inode), start_pos);
 		if (err)
 			return err;
 	}
@@ -1330,7 +1330,8 @@
 		goto out;
 
 	if (alloc_start > inode->i_size) {
-		ret = btrfs_cont_expand(inode, alloc_start);
+		ret = btrfs_cont_expand(inode, i_size_read(inode),
+					alloc_start);
 		if (ret)
 			goto out;
 	}