btrfs: fix cannot use the loop device

we cannot use the loop device which has been connected to a file in the btrf

The reproduce steps is following:

 # dd if=/dev/zero of=vdev0 bs=1M count=1024
 # losetup /dev/loop0 vdev0
 # mkfs.btrfs /dev/loop0
 ...
 failed to zero device start -5

The reason is that the btrfs don't implement either ->write_begin or ->write the VFS API, so we fix it by setting ->write to do_sync_write().

---
 fs/btrfs/file.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

index 79437c5..abcb918 100644
@@ -1197,6 +1197,7 @@ static int btrfs_file_mmap(struct file	*filp, struct vm_area_struct *vma)
 const struct file_operations btrfs_file_operations = {
 	.llseek		= generic_file_llseek,
 	.read		= do_sync_read,
+	.write		= do_sync_write,
 	.aio_read       = generic_file_aio_read,
 	.splice_read	= generic_file_splice_read,
 	.aio_write	= btrfs_file_aio_write,

1.6.5.2